[DOCS] Document site sets of EXT:felogin

This patch adds a description on how to utilize
site sets from `EXT:felogin` and lists the supported
configuration directives, autogenerated from
the settings.definitions.yaml file.

Also some wording changes and typo-fixes in the
settings definitions file are applied, and fe-login
special notices are added.

Resolves: #104597
Releases: main
Change-Id: I14ccef4c2ac5e6d93a39c8a2d41301dd1ab99a3c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85594


Reviewed-by: Chris Müller <typo3@brotkrueml.dev>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Chris Müller <typo3@brotkrueml.dev>
Tested-by: Garvin Hicking <gh@faktor-e.de>