Documentation work

Remove sentence describing GLFW-specific behavior.  This describes the
behavior of glfwUpdateGamepadMappings, whose behavior is a mix between
the two related SDL functions, but is not part of the format.

docs/input.dox

@@ -786,7 +786,6 @@ functions.
 
 There is also the special `platform` field that specifies which platform the
 mapping is valid for.  Possible values are `Windows`, `Mac OS X` and `Linux`.
-Mappings without this field will always be considered valid.
 
 Below is an example of what a gamepad mapping might look like.  It is the
 one built into GLFW for Xbox controllers accessed via the XInput API on Windows.