doc: Add new section describing fast-path locking

doc: Add new section describing fast-path locking

Fast-path locking is referenced by pg_stat_lock.fastpath_exceeded, by
pg_locks.fastpath, and in the GUC max_locks_per_transaction.  However,
the documentation has never described in details how this works; one
would need to look at the internals of lock.c, mostly around
EligibleForRelationFastPath().

This commit adds a new subsection called "Fast-Path Locking" to the area
dedicated to locks, with the three places mentioned above linking to it.

Author: Tatsuya Kawata <kawatatatsuya0913@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAHza6qdKo9dcPy70QBi88vpqhS2gYWViS8=Uj=-+QQbR=ONgSQ@mail.gmail.com