[DOCS] Improve changelog for #97454 - new LinkHandler events

- mention the methods of the removed hook to make it easier
  for developers to search for them.
- add missing description of the ModifyAllowedItemsEvent

Releases: main
Resolves: #97881
Related: #97454
Change-Id: I3667514d8d6b6ef1865e161d77ce18ec2999cca8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75081


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Lina Wolf <112@linawolf.de>