avoid fetching intersphinx inventories over the network all the time
show a warning on docs built from branches other than master
don't treat sphinx warnings as errors for branch docs

Change-Id: I8fe9b624fbb5bb5be3ac4dffcb5289785da821c5
Merge changes I8fe9b624,Ic4d4df98,I7bec34e8

* changes:
  don't treat sphinx warnings as errors for branch docs
  show a warning on docs built from branches other than master
  avoid fetching intersphinx inventories over the network all the time