GenAI: clarify that GenAI spans capture logical operation duration and outcome

[lmolkova]

In other semantic conventions, we provide some guidance on what span should represent.

E.g. in database we clarify it's the duration as observed by user whenever it's possible to capture it.

https://github.com/open-telemetry/semantic-conventions/blob/11f5d37d1624c2392ea97f40e76c1b3c7cd30124/docs/database/database-spans.md?plain=1#L49-L51

In practice it means that GenAI instrumentation applies to the public API and protocol level span(s) such as HTTP become children of GenAI spans.