doc: inform the diff between config from zebra and outside

doc: inform the diff between config from zebra and outside

To avoid some confusions, it is precised in the documentation that
the configuration not done from zebra will not be injected in the
configuration context. As consequence, the config file will not be
impacted by underlying network context. But also, this will not be
possible for *Zebra* to attempt to modify outside networking objects.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>