pgsql: 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(at)gmail(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAHza6qdKo9dcPy70QBi88vpqhS2gYWViS8=Uj=-+QQbR=ONgSQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d31da527169fb54916d7435629c04a7b7bbda1d

Modified Files
--------------
doc/src/sgml/config.sgml | 6 ++++++
doc/src/sgml/monitoring.sgml | 3 +++
doc/src/sgml/mvcc.sgml | 38 ++++++++++++++++++++++++++++++++++++++
doc/src/sgml/system-views.sgml | 5 +++--
4 files changed, 50 insertions(+), 2 deletions(-)