Emit warnings when row limits are reached

For instance, the following should emit a warning:

``` python
from astropy.io import fits
table = Irsa.query_region("m42", catalog="fp_psc", spatial="Cone",
radius=10 * u.arcmin)
```

I spent a while figuring out why a patch of sky was missing, and then realized the row limit was quite low (500)

cc @Cadair

Start with the Irsa.query_region example and trace where its row limit is configured and where the returned row count is handled. The issue names no files or tests, so locate the relevant query implementation and existing warning tests first. Done means a query reaching its configured row limit emits a warning.

Written by the indexing model from the issue text.