[TASK] Improve the comments and type annotations of MailUtility

This helps extensions that rely on the type annotations of that
class for static analysis.

Resolves: #97812
Related: #97377
Releases: 11.5
Change-Id: I0b48f459617e4f4e6c609bdb3abf6f8b465dabd9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74985


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