style.mdoc.5: maintenence

• description: increase visibility by s/file/manual page/
• examples: s/No Doing Something/Doing Something/
• examples: remove depreciated .Li macro
• examples: remove extra newline (one display block)
• see also: link roff language reference for mandoc

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1130

(cherry picked from commit e7ff917057eaf209c003660f02decb64ccb811da)