[release-branch.go1.13] net/http: make docs refer to Context.Value as a getter...

[release-branch.go1.13] net/http: make docs refer to Context.Value as a getter instead of context.WithValue

The doc comments of both ServerContextKey and LocalAddrContextKey both suggest that context.WithValue can be used to access (get) properties of the server or connection. This PR fixes those comments to refer to Context.Value instead.

Change-Id: I4ed383ef97ba1951f90c555243007469cfc18d4d
GitHub-Last-Rev: 05bc3acf82322e3dc77abc7fa0412efe01a77eac
GitHub-Pull-Request: golang/go#33833
Reviewed-on: https://go-review.googlesource.com/c/go/+/191838


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 8b03a399)
Reviewed-on: https://go-review.googlesource.com/c/go/+/191750


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>