pgsql: Fix cross-checking ofReservedBackends/max_wal_senders/MaxConnec

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4e0c743c18bf5435a4850510c5c74b3521c3e1e5

Modified Files
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doc/src/sgml/config.sgml            | 23 ++++++++++++++---------
src/backend/postmaster/postmaster.c | 15 ++++++---------
src/backend/utils/init/postinit.c   |  2 +-
src/backend/utils/misc/guc.c        |  5 +++--
4 files changed, 24 insertions(+), 21 deletions(-)