bug#5718: scroll-margin in buffer with small line count.

[Eli Zaretskii]

> From: npostavs@users.sourceforge.net
> Cc: 5718@debbugs.gnu.org,  ahyatt@gmail.com,  gavenkoa@gmail.com
> Date: Sat, 28 Jan 2017 19:57:21 -0500
> 
> Okay, I think this is ready now, I'm posting the final patchset for
> reference.  It's basically the same as the previous stuff, but I've made
> maximum-scroll-margin customizable, and renamed window-tests.el to
> test/manual/scroll-tests.el (since it only works in an interactive
> session).
> 
> I'll push to master in a couple of days.

Thanks, it LGTM.  Would you mind to mention the new option in the user
manual?