doc: Add note about the server name of postgres_fdw_get_connections() returns.

Previously the document didn't mention the case where
postgres_fdw_get_connections() returns NULL in server_name column.
Users might be confused about why NULL was returned.

This commit adds the note that, in postgres_fdw_get_connections(),
the server name of an invalid connection will be NULL if the server is dropped.

Suggested-by: Zhijie Hou
Author: Bharath Rupireddy
Reviewed-by: Zhijie Hou, Fujii Masao
Discussion: https://postgr.es/m/e7ddd14e96444fce88e47a709c196537@G08CNEXMBPEKD05.g08.fujitsu.local

doc/src/sgml/postgres-fdw.sgml

@@ -493,7 +493,9 @@ OPTIONS (ADD password_required 'false');
       each connection is valid or not. <literal>false</literal> is returned
       if the foreign server connection is used in the current local
       transaction but its foreign server or user mapping is changed or
-      dropped, and then such invalid connection will be closed at
+      dropped (Note that server name of an invalid connection will be
+      <literal>NULL</literal> if the server is dropped),
+      and then such invalid connection will be closed at
       the end of that transaction. <literal>true</literal> is returned
       otherwise. If there are no open connections, no record is returned.
       Example usage of the function: