Fixed #9321 -- Deprecated hard-coding of help text in model ManyToManyField fields.

This is backward incompatible for custom form field/widgets that rely
on the hard-coded 'Hold down "Control", or "Command" on a Mac, to select
more than one.' sentence.

Application that use standard model form fields and widgets aren't
affected but need to start handling these help texts by themselves
before Django 1.8.

For more details, see the related release notes and deprecation timeline
sections added with this commit.