net: clarify UDPConn Read behaviour documentation

[kortschak]

The documentation for UPDConn.Read says that "Read implements the Conn Read method." However, UDP is not stream and Conn is described as "a generic stream-oriented network connection".

It would be nice to be able to know what the characteristics of Read are with respect to packets — whether Read only reads a packet at a time or not — without having to read the source.

See https://groups.google.com/forum/#!topic/golang-nuts/f_s3fGRgAF8