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.
Understood, though I don't think this is a case for type casting, as annotating a callback's first argument as `(string|number)` would be incorrect in the cases where it actually is one or the other. Modifying your example:

```js
/** @param {string} str */
function myCallback(str, other) {}
jQuery.each({a:1, b:2, c:3}, myCallback);
```
The compiler would emit a warning if the `jQuery.each` annotation had the callback take a `(number|string)` as a first param. Your proposal would require casting `myCallback` to an incorrect type, which is...meh. (Really, this is not the same as, but a complementary case to the one referenced in the note at the top of the extern file, this one dealing with params instead of return types.)

There's just currently no way to precisely describe the API for this function, along with many other parts of the jQuery API and JS code in general (see #202 for more discussion on this), which is why the "least common denominator" is often used, even in this and other extern files. I'd personally prefer to err on the side of a more permissive annotation than an over-restrictive (and not strictly correct) one.