Add more docs to XMatch

At least 3 more examples need to be added

- [ ] example when the input, uploaded catalog is an astropy Table
- [ ] example to cross match two Vizier catalogs (this needs issue #1474 being fixed)
- [ ] showing examples how to circumvent or avoid the 2million row limit of the server. A new feature in the works partly addresses this, but nevertheless an examples showing the usage should be put in the narrative docs