Add component class help property

The help of a component class is a full text which describes its
purpose, the expected params for its constructor, etc. It does not have
to start with a description since there's already a description property for
this.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>