docs(endpoint): add detail & example to online method

[b5]

Description

learning how to make proper rust docs contributions, clarifying my understanding of what online does, and adding an example.

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[github-actions]

Documentation for this PR has been generated and is available at: https://n0-computer.github.io/iroh/pr/3722/docs/iroh/

Last updated: 2025-12-02T02:32:47Z

[github-actions]

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Last updated for commit: 860bb2f

[flub]

Could you also take #3713 into account and call out that this doesn't interact with the discovery mechanisms?