DB_COMMAND(9): update to mention additional macros

Document the existing alias definitions, and augment the example with
one of these. Also, describe the purpose of the newly added _FLAGS
variations of these command definitions.

Make some small style improvements to appease mandoc -Tlint.

Reviewed by: markj
MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35664