Fixes for setting up scrolling adjustments

StScrollable: Document how to set adjustments
StBoxLayout: Make sure that we always have upper >= lower + page_size,
  so that clamping works properly. Set the page_increment to be slightly
  less than the page_size so there is some overlap, as is customary.
StScrollView: Remove unnecessary fabs() calls, rewrite expressions
  for additional clarity.

https://bugzilla.gnome.org/show_bug.cgi?id=611740