committers-guide: update commit trailer descriptions

• Add Reported by, Tested by, Fixes, MFH.
• Mention that Submitted by is typically not used with git.
• Add more cases to Approved by.
• Move Sponsored by: to the same place it appears in the template.

Reviewed by: debdrup
Approved by: debdrup
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28905