Restore the user-agent-defined edge margin, used to handle overscan

Since commit c2821f7a4170795254403d600350f30f4dc01a3e the reference to
"the 'padding-top' space defined in the next section" is broken.

This restores some of the text removed in commit
3d05e2e05c72b01404cf290bf07c7f0f367827a7

Related to but does not resolve
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25637
Merge the vertical growing left/right cases, for consistency
Merge pull request #143 from foolip/overscan

Restore the user-agent-defined edge margin, used to handle overscan