Fix up the sis.4 man page per Sheldon Hearn:

Fix up the sis.4 man page per Sheldon Hearn:

    *   Clear the Os macro, which is assumed gracefully at run-off time.
    *   Use quotes to reduce the long name description (Nd) to a single
        argument.
    *   Use meaningful arguments to the -width option of the Bl macro.
    *   Mark rc.conf up with Xr instead of Pa so that it is obvious that
        further help on that file is available.
    *   Explicitly indicate that mediaopt is a command modifier (Cm) of
        the ifconfig(8) utility.
    *   Do not mark up half-duplex and full-duplex as arguments (Ar),
        since they are allowed values for an argument.
    *   Fix various grammar and spelling mistakes.