MAST: Missions MAST Followup Issues

Followup issues to PR https://github.com/astropy/astroquery/pull/2095

- [ ] Make ` missions.get_column_list()` return an astropy Table.
- [ ] Add warning when max number of results are returned (as in `mast.catalogues`)
- [ ] Clean up documentation
Fix capitalization and spelling
Make sure all links to functions work
Make quotes around arguments etc consistent with rest of the documentation
Remove redundant text
Add more information about when a user should choose this service
Remove reference to `set_service` and `set_mission` functions (they don't exist anymore)
Make clear what happens when invalid kwargs are passed (that they are just ignored)
- [ ] Add language in the docstring of `services._json_to_table` explaining the possible values of `col_type` and which service(s) they might come from.
- [ ] Explore the `ignore_value` handling in `services._json_to_table`
- [ ] Consider the way the fall through from using idx to col works in `services._json_to_table` (line 80 in services.py)
- [ ] Point user to `missions.get_column_list` function in missions query doc strings