Hinting at the origin_visit_update model

Related to T2310

to prevent sphinx from complaining about these not being included.

text adapted from iPRES 2017 paper about Software Heritage, section "Data
Model"

Closes T787

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.