net: update documentation, example for error values #10624
Description
Suggested in a review of net: fix inconsistent errors (https://go-review.googlesource.com/#/c/9236/):
In a different CL, could you write some documentation about what kinds of errors people can expect, and perhaps an Example function showing how to work with them? See https://groups.google.com/d/msg/golang-nuts/iNVNBXdZK8Q/irnIrgLD2q4J for the kinds of issues programmers want to understand.