Describe as "offset in document" instead of "viewport"

The current description of "offset in viewport" is not intuitive to me.

The position returned is relative to the document, not the viewport. The viewport is the area visible to the user (i.e., the browser window, see [1]), so it would be expected that the return value changes as the user scrolls the document. This is *not* the case for the value returned by this module. I think "offset in *document*" is a more accurate description.

If you agree, I'd also consider changing the GitHub repository description and publishing to npm as something like *offset-in-document*.

[1] http://www.quirksmode.org/mobile/viewports.html#link9