Documentation work

This adds documentation for the width and height parameters of
glfwGetMonitorWorkarea.

Related to #1322.
diff --git a/include/GLFW/glfw3.h b/include/GLFW/glfw3.h
index 3019b95..2defb6f 100644
--- a/include/GLFW/glfw3.h
+++ b/include/GLFW/glfw3.h
@@ -1942,6 +1942,8 @@
  *  @param[in] monitor The monitor to query.
  *  @param[out] xpos Where to store the monitor x-coordinate, or `NULL`.
  *  @param[out] ypos Where to store the monitor y-coordinate, or `NULL`.
+ *  @param[out] width Where to store the monitor width, or `NULL`.
+ *  @param[out] height Where to store the monitor height, or `NULL`.
  *
  *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
  *  GLFW_PLATFORM_ERROR.