When securelevel is >= 1, /dev/io may not be opened at all (not just for write).  Update the securelevel description to match this.
When securelevel is >= 1, /dev/io may not be opened at all (not just for
write).  Update the securelevel description to match this.

Reported by:	Alexandros Kosiaris <akosiaris@gmail.com>
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase