Add note about how each partition's default value is treated, into the doc.

Column defaults may be specified separately for each partition.
But INSERT via a partitioned table ignores those partition's default values.
The former is documented, but the latter restriction not.
This commit adds the note about that restriction into the document.

Author: Fujii Masao
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAHGQGwEs-59omrfGF7hOHz9iMME3RbKy5ny+iftDx3LHTEn9sA@mail.gmail.com

doc/src/sgml/ref/create_table.sgml

@@ -539,6 +539,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       <literal>CHECK</literal> constraints; additional constraints with the
       same name and condition as in the parent will be merged with the parent
       constraint.  Defaults may be specified separately for each partition.
+      But note that a partition's default value is not applied when inserting
+      a tuple through a partitioned table.
      </para>
 
      <para>