types(schematypes): use DocType for instance method this

[pshaddel]

Fixes this issue: #13800

[pshaddel]

Right now instance method names does not have a typing(also the function we are passing) when user is passing InstanceMethod type. Let me know if I need to implement this too.

[ozzyonfire]

This looks great. Thanks for implementing it so quickly. I'm wondering if "this" should also expose the methods of a HydratedDocument. Like for example, "populate".

There are a couple times I call this.populate('item') or something similar inside my instance method. And right now Typescript doesn't like that.

[pshaddel]

Thanks for pointing that out. Just changed it to HydratedDocument.

[pshaddel]

I have pushed a new commit to fix the typescript failing test.

[vkarpov15]

Thanks!