Nghiauet add comment for clarify

[Nghiauet]

Hi, I am reading the paper and trying to reimplement the code. But some functions seem to lack descriptions and hard to understand what going on. What change: Add some comments for perfect_align and is_sent_ok functions What impact: Don't change the logic code.

I still want to add more comments in the code. Please provide your instructions for me to make better explain for this code. Thanks