[TASK] Update manual with new hook

Add information about forgotPasswordMail hook

Change-Id: I6459aa462c5498c468462d4d163abd905f7fc5b3
Resolves: #34234
Releases: 4.7
Reviewed-on: http://review.typo3.org/9193
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert