[Maktaba integration] Document variable types based on maktaba#ensure calls

In keeping with the spirit of DRY, it'd be nice if vimdoc could understand maktaba's variable type checking. Whenever `maktaba#ensure` calls are present for `a:foo`, it could add some (optional) information about the type of foo. It could even document the allowable values when appropriate (this would be really handy for enum-like types).

This could be orthogonal to explicitly annotating types in the vimdoc. We could do one or the other, and if we implement both we could just use whichever is provided and warn for conflicts.