Document `ThisType<T>`

[trusktr]

Continuing from #649, there doesn't seem to be a link to the "Utility Types" page in the sidebar menu, so it is hidden.

[Retsam]

Yup, there's already #994 tracking this.

[karlhorky]

@trusktr I think this can be closed, can't it? Since it's a dupe of #994?

[orta]

Yep @karlhorky just sent the fix and it's being deployed now