[BUGFIX] Update ExtensionUtility::registerPlugin() icon parameter docs

The icon parameter is passed through to TCA "select" items, thus aside
from an icon file path an icon identifier can be used here.

Resolves: #82351
Releases: master, 8.7
Change-Id: Ia84aa9c739c10372f1ad76c5b4dde9f0b1a62c99
Reviewed-on: https://review.typo3.org/53987


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>