src: Document autostart for session demon

The autostart under session daemon might not behave as you'd
expect it to behave. This patch is inspired by latest
libvirt-users discussion:

https://www.redhat.com/archives/libvirt-users/2018-December/msg00047.htmlSigned-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/libvirt-domain.c

@@ -6693,6 +6693,11 @@ virDomainCreateWithFiles(virDomainPtr domain, unsigned int nfiles,
  * configured to be automatically started when the host
  * machine boots.
  *
+ * Please note that this might result in unexpected behaviour if
+ * used for some session URIs. Since the session daemon is started
+ * with --timeout it comes and goes and as it does so it
+ * autostarts domains which might have been shut off recently.
+ *
  * Returns -1 in case of error, 0 in case of success
  */
 int