[TASK] Backend toolbar interface documentation

Add .rst file for toolbar refactoring works and hint about
future changes of checkAccess() method.

Releases: master
Resolves: #63431
Change-Id: Ibbe019a4e204d9e99e0f722630395c010520ee8e
Reviewed-on: http://review.typo3.org/34780
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>