Log which keys were set to nil in deep_munge

deep_munge solves CVE-2013-0155 security vulnerability, but its
behaviour is definately confuisng. This commit adds logging to deep_munge.
It logs keys for which values were set to nil.

Also mentions in guides were added.
Merge pull request #13813 from lukesarnacki/log-deep-munge

Log for which keys values were set to nil in deep_munge