docs/qapi-code-gen: update to cover trace events code generation

docs/qapi-code-gen: update to cover trace events code generation

Previous commits enabled trace events generation for most of QAPI
generated code (except for tests/ and qga/). Let's update documentation
to illustrate it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20220126161130.3240892-6-vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>