doc: better document NetEventsInterface and the deletion of "CNode"s

[vasild]

Document the requirements around the NetEventsInterface's methods and the lifetime of CNode objects in CConnman::m_nodes.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32278.

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Stale ACK	w0xlt

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LLM Linter (✨ experimental)

Possible typos and grammar issues:

• no-referenced -> unreferenced [“no-referenced” is not a valid adjective; use “unreferenced” (or “no longer referenced”) to convey that the CNode has no references]

^{drahtbot_id_5_m}

[vasild]

1091faf8ff...0f26ac35e2 and 0f26ac35e2...e44e669378: further elaborate how we safely access CNodes in m_nodes and how we avoid CNode destruction while it is being referenced.

[vasild]

e44e669378...110548c9ea: take suggestions