[release-branch.go1.15] doc/go1.15: include behavior updates to the context package

Fixes #40737

Change-Id: I8e2c1e1653d427af1ded6d61df1aa450e3c4d35c
Reviewed-on: https://go-review.googlesource.com/c/go/+/248329


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit b2353174)
Reviewed-on: https://go-review.googlesource.com/c/go/+/248331


Reviewed-by: Andrew Gerrand <adg@golang.org>