doc(net): Explain how we prioritise peer messages, and why it is secure

[teor2345]

Motivation

The auditors asked about a TODO about peer starvation.

Closes #6344

Complex Code or Requirements

This PR documents concurrent code. If it's unclear, please ask questions or suggest changes in the PR.

Solution

• Explain why we want to prefer inbound messages when waiting for a request
• Explain why we prefer cancellations and timeouts when waiting for a response

Review

This is a routine audit fix.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #6488 (25b68a7) into main (3d11a09) will increase coverage by 0.04%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6488      +/-   ##
==========================================
+ Coverage   77.80%   77.84%   +0.04%     
==========================================
  Files         304      304              
  Lines       39678    39686       +8     
==========================================
+ Hits        30870    30894      +24     
+ Misses       8808     8792      -16     

[dconnolly]

🎉