DOC: make signatures more explicit in MAST docstrings

I think we can do better that `*args` and `**kwargs` for the Mast methods. E.g.

```
In [14]: MastMissions.query_region?
Signature: MastMissions.query_region(self, *args, **kwargs)
Docstring:
Queries the service and returns a table object.

Given a sky position and radius, returns a list of matching dataset IDs.

Parameters
----------
coordinates : str or `~astropy.coordinates` object
The target around which to search. It may be specified as a
string or as the appropriate `~astropy.coordinates` object.
radius : str or `~astropy.units.Quantity` object, optional
Default 3 degrees.
The string must be parsable by `~astropy.coordinates.Angle`. The
appropriate `~astropy.units.Quantity` object from
`~astropy.units` may also be used. Defaults to 3 arcminutes.
limit : int
Optional and default is 5000.
the maximun number of dataset IDs in the results.
offset : int
Optional and default is 0
the number of records you wish to skip before selecting records.
**kwargs
Other mission-specific keyword args.
Any invalid keys are ignored by the API.
All valid key names can be found using `~astroquery.mast.missions.MastMissionsClass.get_column_list`
function.
For example one can specify the output columns(select_cols) or use other filters(conditions)

Returns
-------
table : A `~astropy.table.Table` object.
```

cc @jaymedina

Start with the MastMissions.query_region docstring and inspect the related Mast method docstrings and MastMissionsClass.get_column_list reference. Update the documented signatures and parameter descriptions so they expose the supported arguments instead of only *args and **kwargs, then verify the generated documentation shows the explicit signatures.

Written by the indexing model from the issue text.