Doc: correct aggressive vacuum threshold for multixact members storage

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 c552e171d16e, so backpatch to
version 14.

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