[TASK] Provide description for "canonical_url" field

- Short description of the function
- Hint that TYPO3 sets default values
- Hint that setting a value will remove the page from the XML sitemap

The behaviour can be changed by the integrator/developer of cause, but
the label can be changed via TCA override in that case.

Resolves: #101841
Releases: main, 12.4
Change-Id: Ic74e02fa4de4aa0a6708123caa596271b7b27561
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80899


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>