net: reference the correct RFCs and sections for IP.IsPrivate

Properly cite RFC 1918 Section 3 for ipv4,
and RFC 4193 Section 8 for ipv6 comments.

Updates #29146

Change-Id: I8a2df0d7bef50444294bb3301fe09fb09f21ffaf
GitHub-Last-Rev: b0341791c0c4a2f47fcea65a7ab3877afbe2040a
GitHub-Pull-Request: golang/go#45500
Reviewed-on: https://go-review.googlesource.com/c/go/+/309249


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>