Clarify spec: Add definitions to Cause and Effect values and clarify Trip Update precedence for consumer routing decisions #648

[Skander Chouchene]

A pull request has been opened to Add definitions to Cause and Effect values and clarify Trip Update precedence for consumer routing decisions. These changes are intended to clarify definitions and to clarify a principle for expected consumer behaviour.

Link: https://github.com/google/transit/pull/648