pgsql: Improve errhint() about replication slot naming restrictions.

Improve errhint() about replication slot naming restrictions.

The existing hint talked about "may only contain letters", but the
actual requirement is more strict: only lower case letters are allowed.

Reported-By: Rushabh Lathia
Author: Rushabh Lathia
Discussion: AGPqQf2x50qcwbYOBKzb4x75sO_V3g81ZsA8+Ji9iN5t_khFhQ@mail.gmail.com
Backpatch: 9.4-, where replication slots were added

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/920218cbc0a36f18793c20184b7ebb999d28e8c8

Modified Files
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contrib/test_decoding/expected/ddl.out |    2 +-
src/backend/replication/slot.c         |    2 +-
2 files changed, 2 insertions(+), 2 deletions(-)