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

contrib/test_decoding/expected/ddl.out

@@ -12,7 +12,7 @@ ERROR:  replication slot "regression_slot" already exists
 -- fail because of an invalid name
 SELECT 'init' FROM pg_create_logical_replication_slot('Invalid Name', 'test_decoding');
 ERROR:  replication slot name "Invalid Name" contains invalid character
-HINT:  Replication slot names may only contain letters, numbers, and the underscore character.
+HINT:  Replication slot names may only contain lower case letters, numbers, and the underscore character.
 -- fail twice because of an invalid parameter values
 SELECT 'init' FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', 'frakbar');
 ERROR:  could not parse value "frakbar" for parameter "include-xids"

src/backend/replication/slot.c

@@ -195,7 +195,7 @@ ReplicationSlotValidateName(const char *name, int elevel)
 					(errcode(ERRCODE_INVALID_NAME),
 			errmsg("replication slot name \"%s\" contains invalid character",
 				   name),
-					 errhint("Replication slot names may only contain letters, numbers, and the underscore character.")));
+					 errhint("Replication slot names may only contain lower case letters, numbers, and the underscore character.")));
 			return false;
 		}
 	}