pgsql: doc: Correct the documented type of pg_get_multixact_stats().num

doc: Correct the documented type of pg_get_multixact_stats().num_mxids

The documentation listed num_mxids as integer, but its SQL type has
been bigint since the function was introduced. Correct the
documentation to match the actual return type.

Backpatch to v19, where the function was introduced.

Author: Naeem Akhter <naeem(dot)akhter(at)percona(dot)com>
Author: Noriyoshi Shinoda <noriyoshi(dot)shinoda(at)hpe(dot)com>
Reviewed-by: Naga Appani <nagnrik(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAKB0-TEcYJoUn8Q_nug8KEvC9Ea0FV_JOe8XwcW-9QrqUjaBxQ@mail.gmail.com
Discussion: https://postgr.es/m/LV8PR84MB3787CF6E56DCC99AE23E712EEE242@LV8PR84MB3787.NAMPRD84.PROD.OUTLOOK.COM
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/599e012921631fa99ccac7edb215dcd437ef0b25

Modified Files
--------------
doc/src/sgml/func/func-info.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)