Miscellaneous doc string improvements

[tomdonaldson]

While implementing a few queries I stumbled across some corrections/improvements that could be made to the in-code documentation. No doubt there are plenty of others, and a thorough review wouldn't be a bad idea, but here is a start:

https://github.com/astropy/pyvo/blob/96443beb9e623bd2ff5a2cbb143001188dfbef37/pyvo/registry/regtap.py#L330

• Replace the reference to a nonexistent 'database' search with a reference to TAP.
• Says that self.search() is equivalent to self.to_service().search(), but to_service() doesn't exist. It should be just service(), which is a little inconsistent with RegistryResults where table() has been deprecated in favor of to_table().

https://github.com/astropy/pyvo/blob/96443beb9e623bd2ff5a2cbb143001188dfbef37/pyvo/dal/query.py#L426

• Typo in deprecation warning.

https://github.com/astropy/pyvo/blob/96443beb9e623bd2ff5a2cbb143001188dfbef37/pyvo/dal/sia.py#L188

• Has 2 different descriptions for size argument, neither of which are very clear about what you can/should supply. (This function doc is repeated multiple places in sia.py.)
• Documents the intersect argument which is not required for SIA services and probably isn't well-supported. The docs should at least have that caveat.

https://github.com/astropy/pyvo/blob/96443beb9e623bd2ff5a2cbb143001188dfbef37/pyvo/dal/query.py#L291

• votable arg is listed as a string but is really a astropy.io.votable.tree.VOTableFile

[bsipocz]

Would be nice to get the docs eventually cleaned up, maybe something for the next release? (This could also go in a bugfix one...)