Remove 'Hold down "Control"...' messages from many field help_text

When getting the `help_text` from a ManyToManyField, Django appends 'Hold down "Control", or "Command" on a Mac, to select more than one.' to the `help_text`. This makes some sense in the browseable API, but no sense when making an OPTIONS request to the API.

It should, at a minimum be removed in the response to OPTIONS requests and possibly from the browseable API. Thoughts? I can come up with a pull request for this if you think it's something DRF should do.