to use the opcodedesc environment.

Changed a \code{} to a \file{} near the start where a file is referenced.

Fixed a typo:  "on" --> "one" in ROT_THREE description.

Wherever opcodes were referenced by name, use \code{OPCODE_NAME}; usage was
inconsistent.  Ideally, another macro would be defined since these don't
represent code a programmer would type, but that's minor even for me.  It'll
probably get converted automatically in the SGML conversion project.  Whether
that matters or not isn't relevant at this point.