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