[docs] [issue17006] Add advice on best practices for hashing secrets

Raymond Hettinger added the comment:

So probably it should stay a warning in this particular case.

Please don't. Python's docs have become cluttered with warning and danger signs. This stands in marked contrast with the docs for other languages which are much cleaner. Our docs have also started to become "preachy", telling people how we think they should write programs rather than documenting what the language does. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17006> _______________________________________