WIP: ERP5Type: Allow overriding _getAcquireLocalRoles .

Put auto-generated _getAcquireLocalRoles at the end of mro. Also, document why it is treated specially here.

Marked "WIP":

• I did not run all tests on this yet (currently on for_testrunner_2 branch)
• @seb: please confirm this fixes the regression you found (done)

/cc @arnau