Misc #215: Reformat usage text & update docs

 - Reformat usage text by adding a blank line between descriptions of
   two switches. (#215)

 - Update documentation for a bit. Correct some wrong statements and
   clarify some others.
Bug fix #244: Build failure with -DDEBUG_EVENTS

Fix build failure with -DDEBUG_EVENTS, caused by incorrect references to
X Sync macros. Thanks to blueyed for reporting. (#244)
Bug fix: --no-fading-openclose doesn't work when fading out

 - Fix the bug that --no-fading-openclose doesn not work when fading out
   by writing w->fade_last as well in win_determine_fade().

 - Write w->fade_last for applying w->fade_force, as well.