Bug: introspection documentation is in section "Tracing commands".

Cause: sub-schema qapi/introspect.json lacks a section header, and
therefore goes into whatever section precedes its include.

Fix: add a section header.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1503602048-12268-3-git-send-email-armbru@redhat.com>