doc: clarify when writable.end callback is called

The current documentation for writable.end only specifies that the callback is called "once the data has been fully handled". It is ambiguous whether this means "successfully handled" and, if so, whether the callback is called if the data can not be successfully handled (i.e. an error occurred).

The ambiguity is not only in the documentation. The stream class implementations differ on this point. stream.Writable invokes the callback with any errors that occur during parameter checking or during calls to _write. However, not all classes return all errors to _write. zlib.Zlib does pass argument and state errors to the _write (_transform) callback, but does not pass data errors. http.OutgoingMessage passes argument type errors and some other types of errors, but not all.

This inconsistency is behind issue #1746 (closed) and, I suspect, other issues in client code which passes a callback to write.

This commit takes no position on whether the callback error behavior should changed, but simply attempts to document the current behavior in a way that is open to changes so that users are not caught by surprise.

Thanks for considering, Kevin