proxy: add mention of RFC 1929 for SOCKS5
proxy: add mention of RFC 1929 for SOCKS5

RFC 1929 describes the SOCKS5 username/password authentication method.
The proxy package implements this method, but doesn't mention RFC 1929.
This change adds mention of RFC 1929.

Change-Id: I30487fb41c1baa16b6ee8a99210168a597e5cb65
Reviewed-on: https://go-review.googlesource.com/60870
Reviewed-by: Mikio Hara <e2f4d2eec1d9a1f781345cdd7260d7d52e6468ae@gmail.com>
Reviewed-by: Avelino <8efd86fb78a56a5145ed7739dcb00c78581c5375@avelino.xxx>
Run-TryBot: Mikio Hara <e2f4d2eec1d9a1f781345cdd7260d7d52e6468ae@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net