MFC jail: make jail(8) man page more readable and more correct

The synopsis section of jail(8) is fine at showing everything that could
be on the command line, but doesn't make much sense. Add some sub-
sections for the different uses of the command.

Also fix up the paragraph about command-line parameter specification,
including removing some clearly erroneous information.

Approved by: re (gjb)
Reviewed by: dvl
Differential Revision: https://reviews.freebsd.org/D41606

(cherry picked from commit dab7fc49e7a65379a0050e8cab0b50afacdc0fc5)
(cherry picked from commit db08e8ba0e7bebbd1af1ee91f0235cd36c9bb7d3)