pgsql: postgres_fdw: Fix error message for PREPARE TRANSACTION.

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

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bca0aed2d01ce50e2f7be14c97f80118b0f67952

Modified Files
--------------
contrib/postgres_fdw/connection.c | 16 ++++++++--------
doc/src/sgml/postgres-fdw.sgml | 6 ++++++
2 files changed, 14 insertions(+), 8 deletions(-)