pgsql: 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(dot)evan(dot)chao(at)gmail(dot)com>
Author: Yingying Chen <cyy9255(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGGTb65Qmtor2nJP-ATgfWpMpD2qhKrdyO7fmRbbS++nQ=vtMw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/30937c60cd9d9069a0377214d9675a7128a2e5a1

Modified Files
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doc/src/sgml/func/func-info.sgml | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)