Fix cursor coordinate phrasing

Fixes #387.

docs/input.dox

@@ -224,8 +224,9 @@ position callback.
 glfwSetCursorPosCallback(window, cursor_pos_callback);
 @endcode
 
-The callback functions receives the cursor position.  On platforms that provide
-it, the full sub-pixel cursor position is passed on.
+The callback functions receives the cursor position, measured in screen
+coordinates but relative to the top-left corner of the window client area.  On
+platforms that provide it, the full sub-pixel cursor position is passed on.
 
 @code
 static void cursor_position_callback(GLFWwindow* window, double xpos, double ypos)

include/GLFW/glfw3.h

@@ -956,8 +956,10 @@ typedef void (* GLFWmousebuttonfun)(GLFWwindow*,int,int,int);
  *  This is the function signature for cursor position callback functions.
  *
  *  @param[in] window The window that received the event.
- *  @param[in] xpos The new x-coordinate, in screen coordinates, of the cursor.
- *  @param[in] ypos The new y-coordinate, in screen coordinates, of the cursor.
+ *  @param[in] xpos The new cursor x-coordinate, relative to the left edge of
+ *  the client area.
+ *  @param[in] ypos The new cursor y-coordinate, relative to the top edge of the
+ *  client area.
  *
  *  @sa @ref cursor_pos
  *  @sa glfwSetCursorPosCallback