doc: Clarify logical replication details

Document more explicitly that the target table can have more columns
than the source table.

Reported-by: Euler Taveira <euler@timbira.com.br>

doc/src/sgml/logical-replication.sgml

@@ -200,7 +200,9 @@
 
   <para>
    Columns of a table are also matched by name.  A different order of columns
-   in the target table is allowed, but the column types have to match.
+   in the target table is allowed, but the column types have to match.  The
+   target table can have additional columns not provided by the published
+   table.  Those will be filled with their default values.
   </para>
  </sect1>