Add monitor and joystick user pointers

docs/monitor.dox

@@ -86,14 +86,16 @@ void monitor_callback(GLFWmonitor* monitor, int event)
 @endcode
 
 If a monitor is disconnected, all windows that are full screen on it will be
-switched to windowed mode.
+switched to windowed mode before the callback is called.  Only @ref
+glfwGetMonitorName and @ref glfwGetMonitorUserPointer will return useful values
+for a disconnected monitor and only before the monitor callback returns.
 
 
 @section monitor_properties Monitor properties
 
 Each monitor has a current video mode, a list of supported video modes,
-a virtual position, a human-readable name, an estimated physical size and
-a gamma ramp.
+a virtual position, a human-readable name, a user pointer, an estimated physical
+size and a gamma ramp.
 
 
 @subsection monitor_modes Video modes
@@ -187,6 +189,17 @@ and make may have the same name.  Only the monitor handle is guaranteed to be
 unique, and only until that monitor is disconnected.
 
 
+@subsection monitor_userptr User pointer
+
+Each monitor has a user pointer that can be set with @ref
+glfwSetMonitorUserPointer and queried with @ref glfwGetMonitorUserPointer.  This
+can be used for any purpose you need and will not be modified by GLFW.  The
+value will be kept until the monitor is disconnected or until the library is
+terminated.
+
+The initial value of the pointer is `NULL`.
+
+
 @subsection monitor_gamma Gamma ramp
 
 The gamma ramp of a monitor can be set with @ref glfwSetGammaRamp, which accepts