swarm/src/handler: Document responsibility limiting inbound streams

[mxinden]

Description

Document that the ConnectionHandler implementation has to enforce a limit on
the number of inbound substreams.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

Thanks for the review @thomaseizinger!