Problem

Some rules in RULES.md are referencing to Google's original python validator implementations, whereas others are referencing to the corresponding part of the spec or best practices.
For a casual validator user, the most useful is to link to the spec or best practice mention, in order to have additional context about the issue.
The notices that are only referencing to the python validator should also link to the corresponding spec or best practices, if existing.

Additional context

#1132 (comment)