[TASK] Clarify docblock for addPlugin

Clarify in the docblock that it is also possible to pass an icon
identifier to addPlugin, not just an icon path.

The first argument of the method ExtensionManagementUtility::addPlugin()
expects an array. The possible values are explained in the docblock. For
the third value, the icon, it is possible to pass a path or an icon
identifier.

Resolves: #97827
Releases: main, 11.5
Change-Id: If37661344dd4112a076b1b26d3a7fe54b2a6a0cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74943


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>