[PATCH] doc: clarify pg_stat_lock.fastpath_exceeded scope

Hi,

While developing the pg_stat_lock improvement proposed in (*1), I
noticed that the documentation of the fastpath_exceeded column in
pg_stat_lock could be more explicit about its scope.

The current documentation describes it as:

Number of times a lock of this type could not be acquired via fast
path because the fast path slot limit was exceeded. Increasing
max_locks_per_transaction can reduce this number.

This omits the fact that fast-path locking only applies to relation
locks acquired in the weak modes (AccessShareLock, RowShareLock,
RowExclusiveLock), so the counter is always zero for any other
locktype.

The attached patch adds a short clarification to the pg_stat_lock entry
in monitoring.sgml.

(*1) [PATCH] pg_stat_lock: add blocker mode dimension
https://www.postgresql.org/message-id/flat/CAHza6qeixfLeM3AqEkjgkE%2BBAqxB%3D5p227fo6JR60cG4mj8O8A%40mail.gmail.com#3592fb428248b200452b900468dc6d65

Regards,
Tatsuya Kawata