Add missing error to glfwGetClipboardString docs

Add GLFW_FORMAT_UNAVAILABLE to the list of possible errors in the
reference documentation for glfwGetClipboardString.

Slightly edited by @elmindreda.

Closes #1998

(cherry picked from commit bb193325cc4e5af7d4e715072a242c1d3cfce26e)

include/GLFW/glfw3.h

@@ -5316,8 +5316,8 @@ GLFWAPI void glfwSetClipboardString(GLFWwindow* window, const char* string);
  *  @return The contents of the clipboard as a UTF-8 encoded string, or `NULL`
  *  if an [error](@ref error_handling) occurred.
  *
- *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
- *  GLFW_PLATFORM_ERROR.
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED, @ref
+ *  GLFW_FORMAT_UNAVAILABLE and @ref GLFW_PLATFORM_ERROR.
  *
  *  @pointer_lifetime The returned string is allocated and freed by GLFW.  You
  *  should not free it yourself.  It is valid until the next call to @ref