content models for Atom, Expr and Plex repaired with temporary patch

Eventually, the modules for Atom, Expr and Plex will be included,
just like it is done for Equal now. However, until that is done
the content model, from the documentation, is used directly
to overcome validation problems.

At this point, all examples validate, overlooking XInclude problems.

Signed-off-by: greenTara <taraathan@gmail.com>