Update documentation for span/3

[stefanchrobot]

It wasn't really clear to me how SpanFunction works.

[stefanchrobot]

%% When providing `StartMetadata' and `StopMetadata', these values will be sent independently to `start' and
%% `stop' events. If an exception occurs, exception metadata will be merged onto the `StartMetadata'. In general,
%% `StopMetadata' should only provide values that are additive to `StartMetadata' so that handlers, such as those
%% used for metrics, can rely entirely on the `stop' event.

This section is unclear to me. Can this be reworded into "stop metadata should be a superset of start metadata"?

[stefanchrobot]

@josevalim sorry for the trouble, pushed a minor reword.

[stefanchrobot]

@josevalim can you please have another look?

[josevalim]

💚 💙 💜 💛 ❤️