Document the way partition MERGE/SPLIT operations create new partitions

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/ZilrByTp-pbz6Mvf%40pryzbyj2023
Reviewed-by: Justin Pryzby

Author: akorotkov

doc/src/sgml/ref/alter_table.sgml

@@ -1153,6 +1153,12 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       splitting we have a partition with the same name).
       Only simple, non-partitioned partition can be split.
      </para>
+     <para>
+      The new partitions will be created the same as tables created with the
+      SQL command <literal>CREATE TABLE <replaceable class="parameter">partition_nameN</replaceable> (LIKE <replaceable class="parameter">name</replaceable> INCLUDING ALL EXCLUDING INDEXES EXCLUDING IDENTITY)</literal>.
+      The indexes and identity are created later, after moving the data
+      into the new partitions.
+     </para>
      <note>
       <para>
        This command acquires an <literal>ACCESS EXCLUSIVE</literal> lock.
@@ -1213,6 +1219,12 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       can have the same name as one of the merged partitions.  Only simple,
       non-partitioned partitions can be merged.
      </para>
+     <para>
+      The new partition will be created the same as a table created with the
+      SQL command <literal>CREATE TABLE <replaceable class="parameter">partition_name</replaceable> (LIKE <replaceable class="parameter">name</replaceable> INCLUDING ALL EXCLUDING INDEXES EXCLUDING IDENTITY)</literal>.
+      The indexes and identity are created later, after moving the data
+      into the new partition.
+     </para>
      <note>
       <para>
        This command acquires an <literal>ACCESS EXCLUSIVE</literal> lock.