Increase threshold for multixact member emergency autovac to 50%.

Increase threshold for multixact member emergency autovac to 50%.

Analysis by Noah Misch shows that the 25% threshold set by commit
53bb309d2d5a9432d2602c93ed18e58bd2924e15 is lower than any other,
similar autovac threshold.  While we don't know exactly what value
will be optimal for all users, it is better to err a little on the
high side than on the low side.  A higher value increases the risk
that users might exhaust the available space and start seeing errors
before autovacuum can clean things up sufficiently, but a user who
hits that problem can compensate for it by reducing
autovacuum_multixact_freeze_max_age to a value dependent on their
average multixact size.  On the flip side, if the emergency cap
imposed by that patch kicks in too early, the user will experience
excessive wraparound scanning and will be unable to mitigate that
problem by configuration.  The new value will hopefully reduce the
risk of such bad experiences while still providing enough headroom
to avoid multixact member exhaustion for most users.

Along the way, adjust the documentation to reflect the effects of
commit 04e6d3b877e060d8445eb653b7ea26b1ee5cec6b, which taught
autovacuum to run for multixact wraparound even when autovacuum
is configured off.