Wayland: Document when behaviour differs

This should describe all of the Wayland-specific points, mostly in
three categories:
- Missing GLFW features, for example window frame, screensaver
  inhibition, clipboard or file drop.
- Missing protocols, like setting gamma on a monitor or specifying a
  window icon.
- Fundamental incompatibilities with Wayland concepts, like trying to
  handle global positioning of a window in a 2D space, trying to bring
  a window to front, or trying to change the monitor’s mode.

Closes #881.

docs/Doxyfile.in

@@ -202,6 +202,7 @@ ALIASES                = "thread_safety=@par Thread safety\n" \
                          "errors=@par Errors\n" \
                          "glfw3=@par\n__GLFW 3:__" \
                          "x11=__X11:__" \
+                         "wayland=__Wayland:__" \
                          "win32=__Windows:__" \
                          "osx=__OS X:__"