jail.conf.5: minor cleanup (spdx, macros, xrefs)

+ expand see also section with other relevant pages and the handbook
+ tag paths with the path macro so they can be searched with apropos
+ tag spdx, one sentance per line

MFC after: 3 days

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1371

(cherry picked from commit 944c74b7d93c60709b966a5a522110e4fe1038b2)