mrsas.4: Cleaning

+ Rewrite SYNOPSIS for consistency
+ Rewrite HARDWARE for HW Relnotes, and add some stragglers
+ Correct mdoc grammar, making sysctls aproposable and linking xrefs
+ Clean up TODO, cannonicalize to CAVEATS, a standard section
+ Editorial pass, tag SPDX

MFC after: 3 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D52125