[1.6.x] Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.

Backport of de90129070 from master
[1.6.x] Fixed #22859 -- Improved crossDomain technique in CSRF example.

Thanks flisky for the report.

Backport of 0be4d64487 from master