chroot.8: Add examples & clean up

chroot.8: Add examples & clean up

- Sort arguments in synopsis.
- Clarify that it is possible to specify arguments to the command (and that
  they could be passed as further arguments to chroot(1)).
- Standardize the description of the flags.
- Improve formatting (e.g., do not use macros in strings specifying width).
- Add examples.

Reviewed by:	bcr
Approved by:	bcr (doc)
Approved by:	krion (mentor, implicit), mat (mentor, implicit)
Differential Revision:	https://reviews.freebsd.org/D19582