Fixed documentation of disable_constraint_checking

The docstring and base implementation of disable_constraint_checking do not indicate that a return value is expected for proper behavior.
Fixed #12747 -- Made reason phrases customizable.
Merge pull request #1028 from manfre/patch-2

Fixed #20340 - document required return value for disable_constraint_checking
Fixed #20126 -- XViewMiddleware moved to django.contrib.admindocs.middleware