pgsql: Doc: add commentary about cowboy assignment of maintenance_work_

Doc: add commentary about cowboy assignment of maintenance_work_mem.

Whilst working on commit 041e8b95b 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.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/53a49365052026907afff7613929710d1e7f0da0

Modified Files
--------------
src/backend/commands/vacuumparallel.c | 7 +++++++
1 file changed, 7 insertions(+)