ipv6: simplify references to RFCs in package documentation

Change-Id: I8900190d3234d3f8883761b2ae735f2a959b9a26
Reviewed-on: https://go-review.googlesource.com/33252
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv6/doc.go

@@ -9,10 +9,8 @@
 // manipulation of IPv6 facilities.
 //
 // The IPv6 protocol is defined in RFC 2460.
-// Basic and advanced socket interface extensions are defined in RFC
-// 3493 and RFC 3542.
-// Socket interface extensions for multicast source filters are
-// defined in RFC 3678.
+// Socket interface extensions are defined in RFC 3493, RFC 3542 and
+// RFC 3678.
 // MLDv1 and MLDv2 are defined in RFC 2710 and RFC 3810.
 // Source-specific multicast is defined in RFC 4607.
 //