pgsql: doc: Improve description of pg_stat_activity.backend_type

doc: Improve description of pg_stat_activity.backend_type

The documentation of pg_stat_activity used an incomplete list of values
for backend_type. While on it, it is improved to use an itemized list,
now ordered alphabetically, with a short description about each item.

Author: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
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/5e94c0196084f648ae6a00107125494f5804318a.camel@cybertec.at

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/abbd74ce8738d536e8d99151122b7a650e3b63d5

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 165 ++++++++++++++++++++++++++++++++++++++++---
1 file changed, 155 insertions(+), 10 deletions(-)