doc: Clarify GitLab/Gitaly boundary for RPCs

In the past it wasn't always clear where exactly we put the boundary for RPCs that should be implemented in Gitaly versus those that should be in Rails. This was recently clarified though: Gitaly should always avoid to implement business-specific logic and policy. Instead, its provided RPCs should focus on the problem-domains specific to Gitaly and Praefect.

Clarify this in our documentation to make it more readily discoverable by engineers.