doc: added what buf.copy returns
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 <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node