Fix typo in doc
Update intro.doc with info from Gabriel Scherer

Gabriel Scherer <gabriel.scherer@gmail.com> informed us that OCaml
4.02's printf had a makeover and is significantly more efficient.
Indeed it is.
Improve nvector implementation notes

- Mention fewer function names to avoid bit rot (some of them had
  already rotten).
- Correct a reference to a function name.
- Slightly more succinct explanation.