help: document color/pager pitfalls on Windows
Even though I figured this out a few weeks ago, I was initially puzzled where
the color went when I upgraded to 4.2 on a different Windows machine. Let's
point users reading the help into the right direction.
I wonder if we should be even more explicit about cmd.exe/MSYS/pager/color
interplay, but at least all of the breadcrumbs are here (I think).