[TASK] Further improvements for the site configuration

This patch enhances the self declaration of the site
configuration form elements.

- TCA descriptions were added to specific elements to
  provide a better clarity on how to use them.
- First char uppercase on existing descriptions.
- Input fields gets properly trimmed now.
- Additional placeholders were added.

Beside that, all missing PHPDoc blocks are added to
the previously added UserFunctions class.

Releases: master
Resolves: #86387
Related: #85887
Change-Id: I684bb2044e4bb7ce3a6c60bf4656e83260fedafe
Reviewed-on: https://review.typo3.org/58399


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>