Doc: add commentary about cowboy assignment of maintenance_work_mem.

Whilst working on commit 041e8b9 I happened to notice that
parallel_vacuum_main() assigns directly to the maintenance_work_mem
GUC.  This is definitely not per project conventions, so I tried to
fix it to use SetConfigOption().  But that fails with "parameter
cannot be set during a parallel operation".  It doesn't seem worth
working on a cleaner answer, at least not till we have a few more
instances of similar problems.  But add some commentary, just so
nobody gets the idea that this is an approved way to set a GUC.

Author: tglsfdc

src/backend/commands/vacuumparallel.c

@@ -1032,6 +1032,13 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
 	vac_open_indexes(rel, RowExclusiveLock, &nindexes, &indrels);
 	Assert(nindexes > 0);
 
+	/*
+	 * Apply the desired value of maintenance_work_mem within this process.
+	 * Really we should use SetConfigOption() to change a GUC, but since we're
+	 * already in parallel mode guc.c would complain about that.  Fortunately,
+	 * by the same token guc.c will not let any user-defined code change it.
+	 * So just avert your eyes while we do this:
+	 */
 	if (shared->maintenance_work_mem_worker > 0)
 		maintenance_work_mem = shared->maintenance_work_mem_worker;