Various corrections to jQuery 1.9 externs

- Loosens type of jQuery.each callback's first parameter; common subtype `?` preferred over `(string|number)`.
- Deprecates jQuery.support (see jquery/api.jquery.com#454).
- Adds other possible return values to jQuery.parseJSON (see jquery/api.jquery.com#492).
- Loosens return type of `dataFilter` function passed to $.ajax (see jquery/api.jquery.com#492).
- Updates `@see` reference to link to proper issue in old tracker.

Resolves google/closure-compiler#694.
`string|number` is preferred. `?` should be avoided since we know the type here.