[TASK] Add manual for moved signal "postProcessMirrorUrl"

The signal "postProcessMirrorUrl" was moved while refactoring the
"Language" backend module. Add a Breaking ReST manual to
describe the change and the migration path.

Resolves: #66930
Related: #62983
Releases: master
Change-Id: I751a7c8c49500dd482fabe72c3b842118eb1a7a6
Reviewed-on: http://review.typo3.org/39522


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>