net: use IPv4/IPv6/MAC/EUI reserved address blocks for documentation

[mikioh]

There are reserved address blocks for documentation; https://tools.ietf.org/html/rfc5737 and https://tools.ietf.org/html/rfc3849. It'd be better to use them for documentation updates.

• IPv4 address block for doc: 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24
• IPv6 address block for doc: 2001:db8::/32 (also see https://tools.ietf.org/html/rfc5952)

[gopherbot]

CL https://golang.org/cl/21642 mentions this issue.

[gopherbot]

CL https://golang.org/cl/23126 mentions this issue.

[gopherbot]

CL https://golang.org/cl/31950 mentions this issue.

[gopherbot]

CL https://golang.org/cl/32146 mentions this issue.

[gopherbot]

CL https://golang.org/cl/34431 mentions this issue.

[gopherbot]

CL https://golang.org/cl/42991 mentions this issue.