Add documentation for SpanAttributeKey

I only found this by mistake. It isn't obvious from the reference documentation how you would use this.

I'm assuming it works as follows.
```swift
extension SpanAttribute {
var httpMethod: SpanAttributeKey { .init(name: "http.method") }
}
span.attributes.httpMethod = .string("GET")
```
Is there anyway to get this to work
```swift
func setMethod(_ method: String) {
span.attributes.httpMethod = method
}
```

Start by locating the SpanAttributeKey declaration and its reference documentation in the repository. Review the existing SpanAttribute examples, then document the shown usage and clarify whether assigning a plain String to span.attributes.httpMethod is supported. Done means the reference documentation explains both forms or clearly states the supported alternative.

Written by the indexing model from the issue text.