message: Improve tf.test.is_gpu_available documentation
diff: @@ -1554,6 +1554,11 @@ def is_gpu_available(cuda_only=False, min_cuda_compute_capability=None):
  also return False. Use `tf.test.is_built_with_cuda` to validate if TensorFlow
  was build with CUDA support.
  + For example,
  + >>> gpu_available = tf.test.is_gpu_available()
  + >>> is_cuda_gpu_available = tf.test.is_gpu_available(cuda_only=True)
  + >>> is_cuda_gpu_min_3 = tf.test.is_gpu_available(True, (3,0))
  +
  Args:
  cuda_only: limit the search to CUDA GPUs.
  min_cuda_compute_capability: a (major,minor) pair that indicates the minimum
  