Document jQuery.Animation and jQuery.Animation.prefilter

[gnarf]

https://gist.github.com/gnarf/54829d408993526fe475 contains some draft docs for this, but it needs more.

A good prefilter example might be:

http://jsfiddle.net/y4L35/

[sdenowh]

New URL for the draft doc gist:

https://gist.github.com/gnarf/54829d408993526fe475