Consume docs on TagHelper classes to provide IntelliSense at design time.

[NTaylorMullen]

Right now when writing TagHelpers users consume them in the editor without any added IntelliSense. If we were to discover docs related to TagHelper classes and expose those via TagHelperDescriptors we could provide more information to make tooling easier to use.

I believe we should do this at the Razor level to enable cross-platform scenarios.

[danroth27]

Need to understand the tooling impact.

[NTaylorMullen]

This wont be done for beta5, this is a larger issue that will most likely be post rtm.