freebsd-update.conf.5: Address style issues

• Do not set Os to FreeBSD explicitly. We don't do it in other manual pages.
• Remove macros from the -width specifier.
• Use Xr instead of Cm to refer to the freebsd-update command.
• Address some mandoc lint warnings and use \(em instead of --.
• Wordsmith some paragraphs.
• Add a missing El macro.

MFC after: 1 week

(cherry picked from commit b80bb8b197aed4db48810d67be0239fd029c6465)