Doc: Correct description of amcheck example query.

The amcheck documentation incorrectly claimed that its example query
verifies every catalog index in the database.  In fact, the query only
verifies the 10 largest indexes (as determined by pg_class.relpages).
Adjust the description accordingly.

Backpatch: 10-, where contrib/amcheck was introduced.

doc/src/sgml/amcheck.sgml

@@ -35,7 +35,7 @@
   functions.
  </para>
  <para>
-  <filename>amcheck</filename> functions may be used only by superusers.
+  <filename>amcheck</filename> functions may only be used by superusers.
  </para>
 
  <sect2>
@@ -83,14 +83,13 @@ ORDER BY c.relpages DESC LIMIT 10;
                 | pg_amop_fam_strat_index         |        5
 (10 rows)
 </screen>
-      This example shows a session that performs verification of every
-      catalog index in the database <quote>test</quote>.  Details of just
-      the 10 largest indexes verified are displayed.  Verification of
-      the presence of heap tuples as index tuples is requested for
-      unique indexes only.  Since no error is raised, all indexes
-      tested appear to be logically consistent.  Naturally, this query
-      could easily be changed to call
-      <function>bt_index_check</function> for every index in the
+      This example shows a session that performs verification of the
+      10 largest catalog indexes in the database <quote>test</quote>.
+      Verification of the presence of heap tuples as index tuples is
+      requested for the subset that are unique indexes.  Since no
+      error is raised, all indexes tested appear to be logically
+      consistent.  Naturally, this query could easily be changed to
+      call <function>bt_index_check</function> for every index in the
       database where verification is supported.
      </para>
      <para>
@@ -292,8 +291,7 @@ ORDER BY c.relpages DESC LIMIT 10;
    </listitem>
    <listitem>
     <para>
-     Corruption caused by faulty RAM, and the broader memory subsystem
-     and operating system.
+     Corruption caused by faulty RAM, or the broader memory subsystem.
     </para>
     <para>
      <productname>PostgreSQL</productname> does not protect against correctable