Follow-up on tut and/or FAQ about Cats and tailRecM

[adelbertc]

Currently tut docs just implement tailRecM when implementing Monad which can throw off some beginners who are learning Monad - we should clarify somewhere noticeable why we chose to do what we did.

[kailuowang]

I think this is addressed https://github.com/typelevel/cats/blob/master/docs/src/main/tut/faq.md#tailrecm
Shall we close this?

[kailuowang]

@adelbertc see comment above, shall we close this one?