encoding/json: document that embedded interfaces look like non-embedded ones

Fixes issue 8386.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://codereview.appspot.com/149570043
net/url: document result of String

Fixes issue 8742.

LGTM=bradfitz
R=golang-codereviews
CC=adg, bradfitz, golang-codereviews, iant
https://codereview.appspot.com/155910043
os: make Process.Signal 'process finished' error consistent on Unix

While we're here, fix the implementation of Release on both
Unix and Windows: Release is supposed to make Signal an error.

While we're here, make sure we never Signal pid 0.
(Don't try this at home.)

Fixes issue 7658.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://codereview.appspot.com/152240043
os: recomment MkdirAll

The internal comments are not completely precise about
what is going on, and they are causing confusion.

Fixes issue 8283.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/151460043