Doc: Update documentation for asynchronous execution.
Add a note of caution on the performance of asynchronous execution by
postgres_fdw.  Follow-up for commit
Stephen Frost, a little bit expanded by me.
Discussion:
@@ -401,6 +401,16 @@ OPTIONS (ADD password_required 'false');
A table-level option overrides a server-level option.
The default is <literal>false</literal>.
</para>
+
+ <para>
+ In order to ensure that the data being returned from a foreign server
+ is consistent, <filename>postgres_fdw</filename> will only open one
+ connection for a given foreign server and will run all queries against
+ that server sequentially even if there are multiple foreign tables
+ involved, unless those tables are subject to different user mappings.
+ In such a case, it may be more performant to disable this option to
+ eliminate the overhead associated with running queries asynchronously.
+ </para>
</listitem>
</varlistentry>
