[DOCS] Inconsistent and incomplete documentation around returners

There are (I think) three global options around returners - return, return_config, and return_kwargs.

https://github.com/saltstack/salt/blob/master/doc/ref/cli/salt.rst
Only documents return.

https://github.com/saltstack/salt/blob/master/salt/utils/parsers.py
Documents return, return_config, and return_kwargs. However, the return_config is incorrectly a duplicate of return.

https://github.com/saltstack/salt/blob/master/doc/topics/jobs/index.rst#scheduler-with-returner
Only mentions returner, by example. Also mentions schedule_returner without further details.

https://github.com/saltstack/salt/blob/master/salt/modules/schedule.py
Does not document any of the above, but the code lists them in SCHEDULE_CONF.

https://github.com/saltstack/salt/blob/master/salt/states/schedule.py
Has an example with returner, return_config, and return_kwargs, but does not explain them. There's some implication that return_config refers to a named section somewhere.

https://github.com/saltstack/salt/blob/master/doc/ref/returners/index.rst
Only mentions return, by example.

https://github.com/saltstack/salt/blob/master/salt/returners/__init__.py
From looking at existing returners, get_returner_options appears to be how you are supposed to access the config and kwargs, but doesn't actually mention them. The documentation it does have appears to have multiple errors:

• virtualname: the __virtual__() function often returns either True, or (False, msg), not the value of __virtualname__. This may be a wider problem of modules not implementing __virtual__() correctly.
• ret: the "if" / "if not" cases are identical
• attrs: from reading the code this is actually supposed to be a dict, and it's unclear what the values are supposed to be
• profile_attr: "the corresponding key of __salt__" doesn't make sense. __salt__ is the API loader, not a config structure
• profile_attrs: "... a value is read in the are used to fetch a value ..." - a chunk seems to be missing from the middle of this thought
• defaults: isn't documented

In short, if you're trying to write or use a returner, it's almost impossible to find how these options are supposed to work.
I think the return_config and return_kwargs are actually passed to the returner function via ret["ret_config"] and ret["ret_kwargs"]?

Start by comparing the returner options and documentation in doc/ref/cli/salt.rst, doc/topics/jobs/index.rst, doc/ref/returners/index.rst, salt/utils/parsers.py, salt/modules/schedule.py, salt/states/schedule.py, and salt/returners/init.py. Trace get_returner_options and the ret fields to establish the intended behavior. Done means the options and returner API are documented consistently, with the listed inaccuracies corrected.