Doc: correct aggressive vacuum threshold for multixact members storage

The threshold is two billion members, which was interpreted as 2GB
in the documentation. Fix to reflect that each member takes up five
bytes, which translates to about 10GB. This is not exact, because of
page boundaries. While at it, mention the maximum size 20GB.

This has been wrong since commit c552e171d1, so backpatch to
version 14.

Author: Alex Friedman <alexf01@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/CACbFw60UOk6fCC02KsyT3OfU9Dnuq5roYxdw2aFisiN_p1L0bg@mail.gmail.com
Backpatch-through: 14

doc/src/sgml/maintenance.sgml

@@ -792,10 +792,11 @@ HINT:  Stop the postmaster and vacuum that database in single-user mode.
      As a safety device, an aggressive vacuum scan will
      occur for any table whose multixact-age is greater than <xref
      linkend="guc-autovacuum-multixact-freeze-max-age"/>.  Also, if the
-     storage occupied by multixacts members exceeds 2GB, aggressive vacuum
+     storage occupied by multixacts members exceeds about 10GB, aggressive vacuum
      scans will occur more often for all tables, starting with those that
      have the oldest multixact-age.  Both of these kinds of aggressive
-     scans will occur even if autovacuum is nominally disabled.
+     scans will occur even if autovacuum is nominally disabled. The members storage
+     area can grow up to about 20GB before reaching wraparound.
     </para>
 
     <para>