doc: add examples of creative use of unique expression indexes

Unique expression indexes can constrain data in creative ways, so show
two examples.

Reported-by: Tuomas Leikola

Discussion: https://postgr.es/m/156760275564.1127.12321702656456074572@wrigleys.postgresql.org

Backpatch-through: 9.4

doc/src/sgml/indices.sgml

@@ -705,6 +705,15 @@ CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
 </programlisting>
   </para>
 
+  <para>
+   Expression indexes also allow control over the scope of unique indexes.
+   For example, this unique index prevents duplicate integer values from
+   being stored in a <type>double precision</type>-typed column:
+<programlisting>
+CREATE UNIQUE INDEX test1_uniq_int ON tests ((floor(double_col)))
+</programlisting>
+  </para>
+
   <para>
    If we were to declare this index <literal>UNIQUE</literal>, it would prevent
    creation of rows whose <literal>col1</literal> values differ only in case,
@@ -946,6 +955,16 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target)
     This is a particularly efficient approach when there are few
     successful tests and many unsuccessful ones.
    </para>
+
+   <para>
+    This index allows only one null in the indexed column by using a
+    partial index clause to process only null column values, and using
+    an expression index clause to index <literal>true</literal> instead
+    of <literal>null</literal>:
+<programlisting>
+CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL;
+</programlisting>
+   </para>
   </example>
 
   <para>