Adding API change notes

Please place new portions of api_changes.rst in the next_api_changes directory.

When adding an entry please look at the currently existing files to see if you can extend any of them. If you create a file, name it what_api_changes.rst (ex deprecated_rcparams.rst) with contents following the form:

Brief description of change
---------------------------

Long description of change, justification, and work-arounds to
maintain old behavior (if any).

If you need more heading levels, please use ~~~~ and ++++.