Marking up protocols

In the GNOME documentation, we've begun using systemitem for marking up
protocol names (http, ftp, finger, etc.).  There's no suitable value for
the class attribute, so we're setting the role attribute to "protocol".

Some quick googling indicates that a number of other people have taken
the same approach.  Perhaps protocol should be added to the list of
values for the class attribute.

--
Shaun