[DOCS] Clarify what to avoid in checks on TYPO3_MODE

With regards to
https://typo3.slack.com/archives/C025BQLFA/p1585231067123500 there seems
to be confusion in the paragraph about avoiding checks on TYPO3_MODE if
`defined('TYPO3_MODE') or die();` should be used or if that is forbidden
as well.

Releases: master
Resolves: #90843
Change-Id: If1ef849a72bd0aa5337c5c6e8758e69a2f53164b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63934


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>