DOC: COSMIT: Naive Bayes long url + PEP8 cleanup

* Put long link to IR-book on its own line to make sure it renders as a
  correct link in the docs.

* Put long ValueError messages that were breaking PEP8 compliance of
  the file `naive_bayes.py` into temporary variables.

Signed-off-by: mr.Shu <mr@shu.io>
Merge pull request #3892 from mrshu/mrshu/naive-bayes-small-fixes

DOC: COSMIT: Naive Bayes long url + PEP8 cleanup