[release-branch.go1.15] all: merge master into release-branch.go1.15

5c7748dc doc/go1.15: encoding/json's CL 191783 was reverted
5ff5b3c5 doc/go1.15: remove draft notice
5ae1d62e CONTRIBUTORS: update for the Go 1.15 release
7ad776dd doc/go1.15: document crypto/tls permanent error
a93a4c17 runtime: make nanotime1 reentrant

Updates #40697

Change-Id: Ie39896ee6304544cc9e9c1938bdf176f1dcf8766
Reviewed-on: https://go-review.googlesource.com/c/go/+/247900


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>