Doc: clarify that CREATE TABLE discards redundant unique constraints.

The SQL standard says that redundant unique constraints are disallowed,
but we long ago decided that throwing an error would be too
user-unfriendly, so we just drop redundant ones.  The docs weren't very
clear about that though, as this behavior was only explained for PRIMARY
KEY vs UNIQUE, not UNIQUE vs UNIQUE.

While here, I couldn't resist doing some copy-editing and markup-fixing
on the adjacent text about INCLUDE options.

Per bug #16767 from Matthias vd Meent.

Discussion: https://postgr.es/m/16767-1714a2056ca516d0@postgresql.org

doc/src/sgml/ref/create_table.sgml

@@ -878,15 +878,17 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    <varlistentry>
     <term><literal>UNIQUE</literal> (column constraint)</term>
     <term><literal>UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
-    <optional> INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...]) </optional> (table constraint)</term>
+    <optional> <literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>
 
     <listitem>
      <para>
       The <literal>UNIQUE</literal> constraint specifies that a
       group of one or more columns of a table can contain
-      only unique values. The behavior of the unique table constraint
-      is the same as that for column constraints, with the additional
-      capability to span multiple columns.
+      only unique values. The behavior of a unique table constraint
+      is the same as that of a unique column constraint, with the
+      additional capability to span multiple columns.  The constraint
+      therefore enforces that any two rows must differ in at least one
+      of these columns.
      </para>
 
      <para>
@@ -895,10 +897,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
-      Each unique table constraint must name a set of columns that is
+      Each unique constraint should name a set of columns that is
       different from the set of columns named by any other unique or
-      primary key constraint defined for the table.  (Otherwise it
-      would just be the same constraint listed twice.)
+      primary key constraint defined for the table.  (Otherwise, redundant
+      unique constraints will be discarded.)
      </para>
 
      <para>
@@ -911,10 +913,15 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      <para>
       Adding a unique constraint will automatically create a unique btree
       index on the column or group of columns used in the constraint.
-      The optional clause <literal>INCLUDE</literal> adds to that index
-      one or more columns on which the uniqueness is not enforced.
-      Note that although the constraint is not enforced on the included columns,
-      it still depends on them.  Consequently, some operations on these columns
+     </para>
+
+     <para>
+      The optional <literal>INCLUDE</literal> clause adds to that index
+      one or more columns that are simply <quote>payload</quote>: uniqueness
+      is not enforced on them, and the index cannot be searched on the basis
+      of those columns.  However they can be retrieved by an index-only scan.
+      Note that although the constraint is not enforced on included columns,
+      it still depends on them.  Consequently, some operations on such columns
       (e.g., <literal>DROP COLUMN</literal>) can cause cascaded constraint and
       index deletion.
      </para>
@@ -924,7 +931,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    <varlistentry>
     <term><literal>PRIMARY KEY</literal> (column constraint)</term>
     <term><literal>PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>
-    <optional> INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...]) </optional> (table constraint)</term>
+    <optional> <literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>
     <listitem>
      <para>
       The <literal>PRIMARY KEY</literal> constraint specifies that a column or
@@ -942,27 +949,34 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
 
      <para>
       <literal>PRIMARY KEY</literal> enforces the same data constraints as
-      a combination of <literal>UNIQUE</literal> and <literal>NOT NULL</literal>, but
+      a combination of <literal>UNIQUE</literal> and <literal>NOT
+      NULL</literal>.  However,
       identifying a set of columns as the primary key also provides metadata
       about the design of the schema, since a primary key implies that other
       tables can rely on this set of columns as a unique identifier for rows.
      </para>
 
      <para>
-      <literal>PRIMARY KEY</literal> constraints share the restrictions that
-      <literal>UNIQUE</literal> constraints have when placed on partitioned
-      tables.
+      When placed on a partitioned table, <literal>PRIMARY KEY</literal>
+      constraints share the restrictions previously decribed
+      for <literal>UNIQUE</literal> constraints.
      </para>
 
      <para>
       Adding a <literal>PRIMARY KEY</literal> constraint will automatically
       create a unique btree index on the column or group of columns used in the
-      constraint.  The optional <literal>INCLUDE</literal> clause allows a list
-      of columns to be specified which will be included in the non-key portion
-      of the index.  Although uniqueness is not enforced on the included columns,
-      the constraint still depends on them. Consequently, some operations on the
-      included columns (e.g., <literal>DROP COLUMN</literal>) can cause cascaded
-      constraint and index deletion.
+      constraint.
+     </para>
+
+     <para>
+      The optional <literal>INCLUDE</literal> clause adds to that index
+      one or more columns that are simply <quote>payload</quote>: uniqueness
+      is not enforced on them, and the index cannot be searched on the basis
+      of those columns.  However they can be retrieved by an index-only scan.
+      Note that although the constraint is not enforced on included columns,
+      it still depends on them.  Consequently, some operations on such columns
+      (e.g., <literal>DROP COLUMN</literal>) can cause cascaded constraint and
+      index deletion.
      </para>
     </listitem>
    </varlistentry>