In json module there are dump/dumps methods which internally instantiate encoder class JSONEncoder (or some other user-defined encoder clas).
They look as follows:

json.dump(obj, fp, 
          skipkeys=False,
          ensure_ascii=True,
          check_circular=True, 
          allow_nan=True,
          cls=None,
          indent=None,
          separators=None,
          default=None, 
          **kw)

json.JSONEncoder(skipkeys=False, 
                 ensure_ascii=True,
                 check_circular=True,
                 allow_nan=True,
                 sort_keys=False,
                 indent=None,
                 separators=None,
                 default=None)

Some of dump/dumps arguments are passed to encoder class:
- skipkeys
- ensure_ascii
- check_circular
- allow_nan
- indent
- separators
- default

And it looks that sort_keys is just missing in keyword args in dump/dumps method.
But it still can be passed implicitly using **kw arg.

I would propose to do:
- add explicitly sort_keys keyword arg to dump/dumps methods
- add passing it to encoder class
- and adjust documentation accordingly.