pgsql: Fix autovacuum docs related to widening multixid offsets to 64 b

Fix autovacuum docs related to widening multixid offsets to 64 bits

The explanation of aggressive scans claimed that if the multixact
members grows to 2 billion entries, we launch autovacuum even if it's
nominally disabled. That's no longer true; anti-wraparound vacuum is
only triggered if we're in danger of running out of multixids, we no
longer launch it just to shrink the 'members' SLRU. There's no
wraparound danger with 'members' anymore, so it's no worse than if a
user table becomes very bloated.

We do still adjust the freeze threshold though, so that if autovacuum
runs, whether on schedule or as an emergency autovacuum to avoid
wraparound, it will freeze more aggressively if the members SLRU is
large.

I removed the mention of the 4 billion member threshold, even though
that still exists, because the explanation felt misleading. The
aggressiveness is scaled proportionally between 2 and 4 billion
members. At 4 billion it's as aggressive as it gets, but there's no
sudden cliff.

In the passing, fix some comments. Remove obsolete mention of safe
threshold for member storage triggering autovacuum. Secondly, the
freezing cutoff is adjusted to avoid the *members* SLRU from growing
too large.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://www.postgresql.org/message-id/20260827231757.78.noahmisch@microsoft.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6aa2cefe11841a6bff4dab65860c1a12a3e57146

Modified Files
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doc/src/sgml/maintenance.sgml | 17 +++++++----------
src/backend/access/transam/multixact.c | 6 ++----
2 files changed, 9 insertions(+), 14 deletions(-)