Document `ThisType<T>`

[mattmccutchen]

Document the ThisType<T> marker interface that can be included (via intersection) in the contextual type of an object literal to set the type of this for methods defined in the object literal. It doesn't seem to be documented anywhere but the pull request description.

I'll propose text if I have a chance (not right now).

[mweststrate]

I love this feature. Deserves some documentation indeed :)