Doc: add entry for pg_get_partkeydef().

Other pg_get_XXXdef() functions are documented, so it seems reasonable
to include this as well.

Ian Barwick

Discussion: https://postgr.es/m/CAB8KJ=hb2QZXdgyrrRjPCw++DsrRcui4fKArWabQ+oij+2x=_w@mail.gmail.com

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -23718,6 +23718,23 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_get_partkeydef</primary>
+        </indexterm>
+        <function>pg_get_partkeydef</function> ( <parameter>table</parameter> <type>oid</type> )
+        <returnvalue>text</returnvalue>
+       </para>
+       <para>
+        Reconstructs the definition of a partitioned table's partition
+        key, in the form it would have in the <literal>PARTITION
+        BY</literal> clause of <command>CREATE TABLE</command>.
+        (This is a decompiled reconstruction, not the original text
+        of the command.)
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>