.scrollTop(): document proper use of scrollTop on document body

[leobalter]

Fixes #417

I also have included more text for clarification. (can change it if you want)

[gnarf]

I don't like the wording of:

Note that scrollTop can be used to read, but not write, the document scroll position.

It seems that we might be better off saying something like

If you want to read or write the scrollTop() for the whole document, you should use the body element, not window or document or html.

Not sure my wording is any better, but feels less "archaic"

[gnarf]

Also, does this also apply to scrollLeft()?

[jzaefferer]

Thanks for the PR. Since the original Core issue was reopened, I'm going to close this one for now, until the underlying issue is resolved.