…for user folder encryption

Description

Preparing for per folder/user encryption guide per #1904 discussion by cleaning up and simplifying the mail_crypt guide. Also, not enabling mail_crypt globally doesn't allow doveadm mailbox cryptokey to work: http://dovecot.2317879.n4.nabble.com/doveadm-mailbox-cryptokey-not-found-td58533.html

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes