mdo.1: Document group-related and fine-grained control functionalities

This is a manual page update describing the functionality that was added
to mdo(1) in commit 3ca1e69028ac ("mdo(1): Add support and shortcuts for
fully specifying users and groups"). Please either refer to that commit
or the new manual page's content for more information.

While here:

• Add to the introduction a description of process credentials and some specific vocabulary that is used throughout the page, as well as the relationship between mdo(1) and mac_do(4).
• Update the HISTORY section.
• Add AUTHORS and SECURITY CONSIDERATIONS sections.

Reviewed by: ziaee
MFC after: 1 minute
Sponsored by: The FreeBSD Foundation
Sponsored by: Google LLC (GSoC 2025)
Co-authored-by: Kushagra Srivastava <kushagra1403@gmail.com>
Differential Revision: https://reviews.freebsd.org/D53905