Removing recommendation that removeData() is the same as data("key", undefined).

 The removeData docs said that `data("key", undefined)` and `data("key", null)` is the same as `removeData()`
 - `data("key", undefined)` is treated as a getter rather than a setter
 -  `data("key", null)` isn't the same as removeData(); see http://bugs.jquery.com/ticket/11558#comment:10

I've therefore removed mentions of `data("key", undefined)` from `removeData()`, and added a note to `jQuery.data()` and `data()` about this behaviour. I've updated both the `data()`'s to use the "Anything" type for the data value, rather than object.

I've also removed `data("key", null)` from the `removeData()` page.

I've rejiggled the text on the `removeData()` docs, combining the "When using .removeData("name")... " and "As of jQuery 1.4.3" paragraphs; as they were both saying the same thing in a different way.