[DOCS] Correct information about slug postModifiers

The key `record` might not always contain the full record. When using the
`FormSlugAjaxController`, the record might just contain a sub-set of
fields.

Resolves: #94388
Releases: main, 12.4, 11.5
Change-Id: Ie72d6701ebef4da8af8b039e0da056e28b8d0ecb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84691


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>