[release-branch.go1.14] doc/go1.14: document the change to json.Number decoding

[release-branch.go1.14] doc/go1.14: document the change to json.Number decoding

It might break a program if it was depending on undocumented behavior.
Give a proper heads up.

Updates #37308.

Change-Id: Id65bc70def1138d5506b694329c52250b417ec6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/220418
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/220367
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>