Handbook - kernelconfig: Global review

Changes:

• Make the first paragraph a note
• AsciiDoc tweaks
• Remove stale architectures
• Remove editors paragraph
• Add link to config(5)
• Remove pssection
• Restore PORTS_MODULE variable

Reviewed by: fernape@, karels@, mhorne@
Differential Revision: https://reviews.freebsd.org/D39836