docs,fix: Improve error output and documentation clarity in relay ser…

[crStiv]

Changes

This PR includes several improvements to the relay service:

1. Changed eprint! to eprintln! for error output to ensure proper line breaks in terminal output
2. Replaced placeholder "TBD" documentation with actual service description
3. Enhanced RPC endpoint documentation with URL requirements

Motivation

These changes improve developer experience by:

• Making error messages more readable in the terminal
• Providing clear and accurate documentation about the service's purpose
• Adding specific requirements for the RPC endpoint configuration

Impact

These changes are non-functional improvements that only affect documentation and error output formatting. No behavioral changes are introduced.

[mattsse]

makes sense, ty