Documentation improvements

[Daniel (深度碎片)]

@stas

These need to be improved next:

• freeze_to(-3) is equivalent to unfreeze() , all trainable parameters are ready to train.
• freeze_to(-2) only freeze only a small proportion of conv layers in the (0) Sequential

freeze_to operates only on groups, and not what you wrote above.

Hi @stas
I have followed your guide and made corrections to my previous understanding of freeze_to in a new Kaggle kernel.

Should I add the understanding to freeze_to docs? what do you think?

Thanks!