net: update docs on network interface API

This change documents that the InterfaceAddrs function is less usable on
multi-homed IP nodes because of the lack of network interface
identification information.

Also updates documentation on exposed network interface API.

Fixes #14518.

Change-Id: I5e86606f8019ab475eb5d385bd797b052cba395d
Reviewed-on: https://go-review.googlesource.com/31371


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>