minor #11117 Revamped the main security reference article (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes #11117).

Discussion
----------

Revamped the main security reference article

This documents the missing global security options and cleans up the existing reference. There are still a ton of missing config options ... but I'll fix that in a future PR.

Fixes #11102.

Commits
-------

f2a02af Revamped the main security reference article

Author: javiereguiluz