clean up FieldFile.open documentation

[Tobias McNulty]

The documentation for FieldFile.open claims that it behaves like the standard Python open() method, which is incorrect. It does not return a file descriptor the way the Python method does.

It is also unnecessary to call the open method except to re-open the file or change the mode, since the underlying file is opened implicitly any time the file attribute is accessed.

Finally, I found it surprising that two methods in this class default to also saving the model object (associated with the FileField associated with the FieldFile that is being saved or deleted). I added a louder warning about this -- I would rather be reminded twice than spend time trying to figure out how and why my model instance is being saved before I wanted it to be.

[Tobias McNulty]

Pull request here: ​https://github.com/django/django/pull/6459