Doc admin pour VigiGraph.

Change-Id: I9739bedea456f27dfd9769a40f40f23f45b0c984
Reviewed-on: https://vigilo-dev.si.c-s.fr/review/552
Tested-by: Build system <qa@vigilo-dev.si.c-s.fr>
Reviewed-by: Francois POIROTTE <francois.poirotte@c-s.fr>