context: document that WithValue's key must be comparable

context: document that WithValue's key must be comparable

Also, check it and explode earlier, rather than cryptic failures later.

Change-Id: I319a425f60e2bc9d005a187fbdbd153faa96411c
Reviewed-on: https://go-review.googlesource.com/21799
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>