doc: Improve description of pg_get_multixact_stats()

doc: Improve description of pg_get_multixact_stats()

This addresses two gaps in the documentation:
- The function uses an xid, and was not mentioned as an exception in a
section of the docs related to xid8.
- The function returns NULL if a role does not have the privileges of
pg_read_all_stats.  The execution is not denied.

Author: Chao Li <li.evan.chao@gmail.com>
Author: Yingying Chen <cyy9255@gmail.com>
Discussion: https://postgr.es/m/CAGGTb65Qmtor2nJP-ATgfWpMpD2qhKrdyO7fmRbbS++nQ=vtMw@mail.gmail.com