Resurrects tooltips and declaration descriptions for Xamarin Studio

This resurrects tooltips and declaration descriptions.

There were substantial changes in how these are described in code. This is a bit of an approximation but it seems to work ok, at least the information is visible.

For declaration descriptions, the F# langauge service doesn't really format things in the "right" way to make them look pretty in the signature/summary format that Xamarin Studio seems to like.

Parameter info and overload set descriptions still do not work