[TASK] Clarify iteration for re-processing nested tags

Issue #39261 changed the behavior for processing nested tags.
The iteration is adjusted and commented, to make clear what is
actually happening there... (`$tagConfig` did not have a meaning).

Resolves: #92194
Releases: main, 12.4
Change-Id: I94ddc683bbd1241330cde0d44cb7339afa75da56
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79806


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>