Talk about assigning, rather than generating, new MultiXactIds.

The word "assign" is used in various places internally to describe what
GetNewMultiXactId does, but the user-facing messages have previously
said "generate". For consistency, standardize on "assign," which seems
(at least to me) to be slightly clearer.

Discussion: http://postgr.es/m/CA+TgmoaoE1_i3=4-7GCTtKLVZVQ2Gh6qESW2VG1OprtycxOHMA@mail.gmail.com

src/backend/access/transam/multixact.c

@@ -1020,14 +1020,14 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 			if (oldest_datname)
 				ereport(ERROR,
 						(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
-						 errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database \"%s\"",
+						 errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database \"%s\"",
 								oldest_datname),
 						 errhint("Execute a database-wide VACUUM in that database.\n"
 								 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
 			else
 				ereport(ERROR,
 						(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
-						 errmsg("database is not accepting commands that generate new MultiXactIds to avoid wraparound data loss in database with OID %u",
+						 errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database with OID %u",
 								oldest_datoid),
 						 errhint("Execute a database-wide VACUUM in that database.\n"
 								 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));