Fix doc about pg_get_multixact_stats()

Hi Hacker,

I found a small documentation issue in func-info.smgl:

<para>
The internal transaction ID type <type>xid</type> is 32 bits wide and
wraps around every 4 billion transactions. However,
the functions shown in <xref linkend="functions-pg-snapshot"/>, except
<function>age</function>, <function>mxid_age</function>, and
<function>pg_get_multixact_members</function>, use a
64-bit type <type>xid8</type> that does not wrap around during the life
of an installation and can be converted to <type>xid</type> by casting
if
required; see <xref linkend="transaction-id"/> for details.

Since pg_get_multixact_stats() was added in PG19, it should be added to
this exception list.

Regards,
Yingying Chen