Clarify documentation for current_root_span

[timmc-edx]

The documentation for ddtrace.Tracer.current_root_span is ambiguous regarding whether this is A) the root span of the entire trace, B) the root span of the current process, or C) the service entry span:

• « Returns the root span of the current execution. » -- implies B
• « useful for attaching information related to the trace as a whole » -- implies A

Reading the source code, this line implies B, at least to my eyes.

It would be great if the docs could be made clear on this point.

[emmettbutler]

Thanks for noting this, @timmc-edx. We'll clarify the docs.