Document the fl_sys_menu_bar global Mac OS-specific variable.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9996 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Added member function Fl_Sys_Menu_Bar::update().

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9997 ea41ed52-d2ee-0310-a9c1-e6b18d33e121