[TASK] Clarify constants do not work in constant conditions

Frontend TypoScript 'constants' do support conditions. But
constants can not be used within conditions, just like
constants in general can not be used within constants.

The patch adapts the "new TS parser" ReST to clarify.

Resolves: #103953
Related: #97816
Releases: main, 12.4
Change-Id: I0e21a320582f2f0edbde432fe36767540ab7993b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84497


Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>