[TASK] Writing documentation

Add an important hint, when using DCE content elements in other DCEs.

Relates: #58535
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I7e6301eb6bb3cea7696786acb94596159353ddaf
Reviewed-on: https://review.typo3.org/30396
Reviewed-by: Armin Ruediger Vieweg
Tested-by: Armin Ruediger Vieweg