[protos] Reorder some RPCs

Summary:
Minor cleanup in proto file structure:

• Replaced // ... with /* ... */ for group headers to emphasize them over other comments
• Moved some RPCs between groups
• This also reordered generated code

I'm open to other comment suggestions as well. I prioritized these because of RPC order affecting codegen

Depends on D11379

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11435