Modified VO text based on referee comments

Addressed these referee comments:
- The VO cone-search interface has now almost become obsolete. It is totally superseded by the TAP/ObsTAP protocols which allow for much higher flexibility in queries. I believe that the TAP client support should be prioritized in the future Astropy development.
- Please also add references to the VO standards when you mention them (it is done earlier in the text for VOTable but not in Section 4).

This referee comment is noted but no changes are done based on it:
- The integration of Sampy is absolutely essential as it allows one to seamlessly exchange data with other VO applications and data analysis systems such as IRAF. Even DS9 now supports SAMP which immediately added a lot of very interesting and useful capabilities to it.