[release-branch.go1.15] doc/go1.14: document json.Umarshal map key support of TextUnmarshaler

Document that json.Unmarshal supports map keys whose underlying
types implement encoding.TextUnmarshaler.

Updates #38801.
Fixes #41178.

Change-Id: Icb9414e9067517531ba0da910bd4a2bb3daace65
Reviewed-on: https://go-review.googlesource.com/c/go/+/237857


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 47b45099)
Reviewed-on: https://go-review.googlesource.com/c/go/+/252618


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>