PR#581 Update forbidden for network_id in routes.txt if networks.txt exists too

[Skander Chouchene]

A pull request has been opened to adjust the documentation for rules of coexistence between networks files and network_id in routes.txt. The spec mentions that all network files (route_networks.txt and networks.txt) are forbidden if network_id exists in routes.txt. But it only says that network_id in routes.txt is forbidden only if route_networks.txt exists (it does not mention networks.txt despite that being the consensus back then).

Link: https://github.com/google/transit/pull/581