Add documentation for gRPC message and memory limits #390

[jeffngo]

Context: We added metadata and message size limits to two gRPC services (libs server and platform client) that communicate with a Python plugin.

We want to add documentation to describe:

1. The metadata/message size limits
2. What part of the plugin can contribute to these limits
3. How to tell if the limit was exceeded
4. What to do if the limit was exceeded