[TASK] Document 'defaultValue' handling of form

The form framework supports setting a 'defaultValue' for most
of the form elements. This patch adds some information about
this setting and outlines the difference to the 'prependOptionValue'
setting.

Resolves: #82422
Releases: master, 8.7
Change-Id: Ic3bbe76ebe69ac0655e11ba501d62ee19b2d143f
Reviewed-on: https://review.typo3.org/56991


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>