[jquery] Fix TContext declaration of jQuery.proxy on wrong value'…

…s type. (DefinitelyTyped#29930)

* [jquery] Match parameter names to documentation more closely.

* [jquery] Add documentation to parameters.

* [jquery] Fix `TContext` declaration on wrong value's type.

Declaring `this` as `TContext` on the input function ensures that it can handle having its context changed to `TContext`. Declaring `this` on the output function is not important because the function will be already defined. It can also get in the way when passing a callback to a function whose callback parameter has `this` declared.

* [jquery] Drop constraint from `TContext`.

It's not really necessary and it is possible to pass primitives as the context.