BUG #16860: Documentation: GUC Parameters are not explained

The following bug has been logged on the website:

Bug reference:      16860
Logged by:          Boshomi
Email address:      boshomi@gmail.com
PostgreSQL version: 13.1
Operating system:   all
Description:

I am missing a explanation for the abbreviation "GUC" in:
     in
https://www.postgresql.org/docs/current/sepgsql.html#SEPGSQL-PARAMETERS
  and https://www.postgresql.org/docs/current/pgtrgm.html#id-1.11.7.40.7

A link to https://www.postgresql.org/docs/current/config-setting.html would
be helpful.