make_dev.9: Fix mda_cr typo + polish

The make_dev_args_init structure example said mda_cred, however the
correct form is mda_cr according to the reporter and my git grep.
A new document description has been contributed by jhb. While here,
fix grammar and trivial typos (spacing, colons before examples,
trailing delimiter) and spdx.

MFC after: 3 days
Reported by: jhb (much polish and document description)
Reported by: Evgenii Ivanov on Community Discord
Reviewed by: carlavilla, mhorne, jhb, kib
Approved by: carlavilla, mhorne (mentors)
Differential Revision: https://reviews.freebsd.org/D48516