4d55397500 patch 1 (#16176)

* Add pos_weights < 1,  >1 meaning

The current  weighted_cross_entropy_with_logits docs don't explain practically the relationship of 
`pos_weights > 1`,  `pos_weights < 1` to precision, recall, and class imbalance.

* Update nn_impl.py

The current  weighted_cross_entropy_with_logits docs don't explain practically the relationship of 
`pos_weights > 1`,  `pos_weights < 1` to precision, recall, and class imbalance.