Style fixes in comments

[nblanke]

-make the comment style more consistent by replacing Nothing with N/A
-fix typo in sherlock.py

[sdushantha]

Wouldn't "Nothing" be more correct? Because N/A stands for Not Available and the return value is in a way not available because it is "nothing" :P

I have nothing against this change but just curious to know why "N/A" may be better than "Nothing"

[nblanke]

For a function that returns None, both "N/A" and "Nothing" were used in the docstring.
Since most of the docstrings used "N/A" instead of "Nothing" I replaced "Nothing" with "N/A".
But you're correct, using "Nothing" makes more sense.