su use and documentation is misleading. Please review the following:
# /usr/bin/su -m root -C ; # This was intentional to get usage
Unknown option: `-C'
Usage: _su [ -bcdefilmnqstvVxX ] [ argument ... ].
# /usr/bin/su -h
su: illegal option -- h
usage: su [-] [-flms] [-c class] [login [args]]
The latter is what is documented in the man page, but the actual use is described by the former.
It must be very confusing for newbies (or oldies that look into what they use).