Correct documentation of callback arguments

The described behavior is implemented by jQuery 2.x but not jQuery 1.x. I think this is a bug because the documentation already describes the argument type for these callback functions: see [`$.fn.after`](https://github.com/jquery/api.jquery.com/blob/357f4d1ada950415314ab5a4de596d2b1c727009/entries/after.xml#L26) and [`$.fn.before`](https://github.com/jquery/api.jquery.com/blob/357f4d1ada950415314ab5a4de596d2b1c727009/entries/before.xml#L25). If a maintainer could confirm this, I'd be happy to follow up with a patch for jQuery 1.x.

Commit message:

> Both `$.fn.before` and `$.fn.after` invoke the supplied function with
> two arguments: the numeric index of each element in the set and the
> string markup describing each element's contents. The type of these
> arguments is correctly documented, but the description of the second
> argument is omitted. Include this information based on the documentation
> for similar behavior in `$.fn.append`.