scc.4: Improve acronyms + tag SPDX

+ tag spdx
+ remove duplicated acronym in manual title, reword for consistency
+ expand acronym in HARDWARE for HW Relnotes

MFC after: 3 days
Reviewed by: mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D50528