Improve poor security recommendation in docs

As reported in #9960, the current documentation recommends an insecure practice for
key generation from a password (a single round of SHA-256). The modified documentation
uses ActiveSupport::KeyGenerator to perform proper key stretching.
better document `next_week` functionaility closes #9568
Delegation method bug
Add documentation and test to delegation method that make sure we're
aware that when a delegated object is not nil or false and doesn't
respond to the method it will still raise a NoMethodError exception.
Merge pull request #10347 from lellisga/delegation_bug_documentation

Delegation method bug
formatting
Merge pull request #10351 from JonRowe/update_as_date_next_week_to_document_intent

Propose better documentation for ActiveSupports `next_week` functionaility
Merge pull request #9980 from stouset/patch-1

Improve poor security recommendation in docs

[ci skip]
Module#delegate checks nilness rather that falsehood if :allow_nil is true, and avoids multiple evaluation of the target method

Notes:

1) I hope nilness is a word.

2) See rationale for avoiding multiple evaluation in a comment in the patch, credit goes to @jeremy for pointing out this gotcha in the existing implementation.

3) Embeds a little joke dedicated to @pixeltrix (it could be worse! :D).

References #10347.
adding test for the symbol refs