encoding/json: clarify the map's key type for Unmarshal

encoding/json: clarify the map's key type for Unmarshal

While here, also fix doc link for encoding.TextMarshaler.

Fixes #67495

Change-Id: Ia2a674c5c35b5a849ce8f5eef3d34d165b3195b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/593335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>