doc-models.inception_v3: Add information about expected input size

While using the pretrained inception v3 model I wasnt aware that the input size has to be 299x299, as I figured out after a little bit of try and error and searching.

I hope I didn't miss something in the docs, but if not it might make sense to add a small note in the models.inception_v3 doc that the expected tensor size is different for this model.