Clarify documentation of glfwGetClipboardString

The reference documentation for glfwGetClipboardString did not link to
GLFW_FORMAT_UNAVAILABLE.

Fixes #605.

docs/input.dox

@@ -544,9 +544,14 @@ converted to one, you can retrieve it with @ref glfwGetClipboardString.  See the
 reference documentation for the lifetime of the returned string.
 
 @code
-const char* clipboard = glfwGetClipboardString(window);
+const char* text = glfwGetClipboardString(window);
+if (text)
+    insert_text(text);
 @endcode
 
+If the clipboard is empty or if its contents could not be converted, `NULL` is
+returned.
+
 The contents of the system clipboard can be set to a UTF-8 encoded string with
 @ref glfwSetClipboardString.
 

include/GLFW/glfw3.h

@@ -3079,7 +3079,9 @@ GLFWAPI void glfwSetClipboardString(GLFWwindow* window, const char* string);
 /*! @brief Returns the contents of the clipboard as a string.
  *
  *  This function returns the contents of the system clipboard, if it contains
- *  or is convertible to a UTF-8 encoded string.
+ *  or is convertible to a UTF-8 encoded string.  If the clipboard is empty of
+ *  if its contents cannot be converted, `NULL` is returned and a @ref
+ *  GLFW_FORMAT_UNAVAILABLE error is generated.
  *
  *  @param[in] window The window that will request the clipboard contents.
  *  @return The contents of the clipboard as a UTF-8 encoded string, or `NULL`