Improve caching documentation

There's very little documentation on the caching process, and users sometimes complain about the cache filling up.

In short, we need documentation stating:

- [x] The default location is `~/.astropy/cache/astroquery`
- [x] [get_cache_dir](http://docs.astropy.org/en/stable/api/astropy.config.get_cache_dir.html#astropy.config.get_cache_dir) can be used to find the cache dir
- [x] some documentation about how [clear_download_cache](http://docs.astropy.org/en/stable/api/astropy.utils.data.clear_download_cache.html) can be used to clear the cache, with examples

[Edit]:

- [x] add a repeated troubleshooting section to every module's narrative docs that refers to clearing the cache (and upgrading the version) as possible ways to fix failing queries.
- [x] possibly link the above in the class docstrings, too (along with timeout, row_limit, etc.)