Fix cross-checking of ReservedBackends/max_wal_senders/MaxConnections.
We were independently checking ReservedBackends < MaxConnections and
max_wal_senders < MaxConnections, but because walsenders aren't allowed
to use superuser-reserved connections, that's really the wrong thing.
Correct behavior is to insist on ReservedBackends + max_wal_senders being
less than MaxConnections. Fix the code and associated documentation.
This has been wrong for a long time, but since the situation probably
hardly ever arises in the field (especially pre-v10, when the default
for max_wal_senders was zero), no back-patch.
Discussion: https://postgr.es/m/28271.
1520195491@sss.pgh.pa.us