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.

semantic-conventions/docs/database/database-spans.md

Lines 49 to 51 in 11f5d37

	Span that describes database call SHOULD cover the duration of the corresponding call as if it was observed by the caller (such as client application).
	For example, if a transient issue happened and was retried within this database call, the corresponding span should cover the duration of the logical operation
	with all retries.

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.