doc: added what buf.copy returns

as of https://github.com/nodejs/node/blob/v5.x/src/node_buffer.cc#L555

buf.copy returns the number of bytes copied.

PR-URL: https://github.com/nodejs/node/pull/3555


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>