Correct docs about partitions and EXCLUDE constraints.

In version 17 we added support for cross-partition EXCLUDE
constraints, as long as they included all partition key columns and
compared them with equality (see 8c852ba9a4). I updated the docs for
exclusion constraints, but I missed that the docs for CREATE TABLE
still said that they were not supported. This commit fixes that.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Co-authored-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/c955d292-b92d-42d1-a2a0-1ec6715a2546@illuminatedcomputing.com
Backpatch-through: 17

doc/src/sgml/ref/create_table.sgml

@@ -447,11 +447,6 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       the values in the new row, an error will be reported.
      </para>
 
-     <para>
-      Partitioned tables do not support <literal>EXCLUDE</literal> constraints;
-      however, you can define these constraints on individual partitions.
-     </para>
-
      <para>
       See <xref linkend="ddl-partitioning"/> for more discussion on table
       partitioning.
@@ -1162,6 +1157,18 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       exclusion constraint on a subset of the table; internally this creates a
       partial index. Note that parentheses are required around the predicate.
      </para>
+
+     <para>
+      When establishing an exclusion constraint for a multi-level partition
+      hierarchy, all the columns in the partition key of the target
+      partitioned table, as well as those of all its descendant partitioned
+      tables, must be included in the constraint definition.  Additionally,
+      those columns must be compared using the equality operator.  These
+      restrictions ensure that potentially-conflicting rows will exist in the
+      same partition.  The constraint may also refer to other columns which
+      are not a part of any partition key, which can be compared using any
+      appropriate operator.
+     </para>
     </listitem>
    </varlistentry>