Documentation cleanup in response to ML discussion.

Documentation cleanup in response to ML discussion.

 [v0] Accepted with modifications by phcoder@
 [v1] Introduce subsections within Security
 [v1] Correct errors regarding public key files not being automatically signature-checked in trust and verify_detached
 [v1] Replace check_signatures=enforce with check_signatures set to enforce
 [v1] Move detailed discussion of using signatures out of check_signatures environment variable description
 [v1] Use long form for option flags to security-relevant commands
 [v2] Explain the key fingerprint format for distrust and list_trusted.
 [v2] Eliminates references to grub-mkimage and UEFI Secure Boot.
 [v3] Updates in response to addition of --skip-sig to trust and verify_detached
 [  ] Restore @xref for cross-references at the start of sentences