Add statement to docs for glfwTerminate

It is safe to call glfwTerminate both before initialization and after
termination.

Closes #1698.

include/GLFW/glfw3.h

@@ -1881,6 +1881,8 @@ GLFWAPI int glfwInit(void);
  *  call this function, as it is called by @ref glfwInit before it returns
  *  failure.
  *
+ *  This function has no effect if GLFW is not initialized.
+ *
  *  @errors Possible errors include @ref GLFW_PLATFORM_ERROR.
  *
  *  @remark This function may be called before @ref glfwInit.