postgres_fdw: Fix error message for PREPARE TRANSACTION.

Currently, postgres_fdw does not support preparing a remote transaction
for two-phase commit even in the case where the remote transaction is
read-only, but the old error message appeared to imply that that was not
supported only if the remote transaction modified remote tables.  Change
the message so as to include the case where the remote transaction is
read-only.

Also fix a comment above the message.

Also add a note about the lack of supporting PREPARE TRANSACTION to the
postgres_fdw documentation.

Reported-by: Gilles Darold
Author: Gilles Darold and Etsuro Fujita
Reviewed-by: Michael Paquier and Kyotaro Horiguchi
Backpatch-through: 9.4
Discussion: https://postgr.es/m/08600ed3-3084-be70-65ba-279ab19618a5%40darold.net

contrib/postgres_fdw/connection.c

@@ -728,17 +728,17 @@ pgfdw_xact_callback(XactEvent event, void *arg)
 				case XACT_EVENT_PRE_PREPARE:
 
 					/*
-					 * We disallow remote transactions that modified anything,
-					 * since it's not very reasonable to hold them open until
-					 * the prepared transaction is committed.  For the moment,
-					 * throw error unconditionally; later we might allow
-					 * read-only cases.  Note that the error will cause us to
-					 * come right back here with event == XACT_EVENT_ABORT, so
-					 * we'll clean up the connection state at that point.
+					 * We disallow any remote transactions, since it's not
+					 * very reasonable to hold them open until the prepared
+					 * transaction is committed.  For the moment, throw error
+					 * unconditionally; later we might allow read-only cases.
+					 * Note that the error will cause us to come right back
+					 * here with event == XACT_EVENT_ABORT, so we'll clean up
+					 * the connection state at that point.
 					 */
 					ereport(ERROR,
 							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-							 errmsg("cannot prepare a transaction that modified remote tables")));
+							 errmsg("cannot PREPARE a transaction that has operated on postgres_fdw foreign tables")));
 					break;
 				case XACT_EVENT_PARALLEL_COMMIT:
 				case XACT_EVENT_COMMIT:

doc/src/sgml/postgres-fdw.sgml

@@ -468,6 +468,12 @@
    COMMITTED</> local transaction.  A future
    <productname>PostgreSQL</productname> release might modify these rules.
   </para>
+
+  <para>
+   Note that it is currently not supported by
+   <filename>postgres_fdw</filename> to prepare the remote transaction for
+   two-phase commit.
+  </para>
  </sect2>
 
  <sect2>