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-
ections for the different uses of the command.

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

Reviewed by: dvl
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41606