net: make IP.{String,MarshalText} return helpful information on address error

net: make IP.{String,MarshalText} return helpful information on address error

This change makes String and MarshalText methods of IP return a
hexadecial form of IP with no punctuation as part of error
notification. It doesn't affect the existing behavior of ParseIP.

Also fixes bad shadowing in ipToSockaddr and makes use of reserved
IP address blocks for documnetation.

Fixes #15052.
Updates #15228.

Change-Id: I9e9ecce308952ed5683066c3d1bb6a7b36458c65
Reviewed-on: https://go-review.googlesource.com/21642
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>