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: #3555
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.markdown

@@ -358,6 +358,8 @@ if the target memory region overlaps with the source. If `undefined` the
 `targetStart` and `sourceStart` parameters default to `0` while `sourceEnd`
 defaults to `buffer.length`.
 
+Returns the number of bytes copied.
+
 Example: build two Buffers, then copy `buf1` from byte 16 through byte 19
 into `buf2`, starting at the 8th byte in `buf2`.