encoding/json: clarify Unmarshal behavior for map keys

This is a documentation-only change

Fixes #33298

Change-Id: I816058a872b57dc868dff11887214d9de92d9342
Reviewed-on: https://go-review.googlesource.com/c/go/+/188821Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/decode.go

@@ -72,7 +72,8 @@ import (
 // use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
 // reuses the existing map, keeping existing entries. Unmarshal then stores
 // key-value pairs from the JSON object into the map. The map's key type must
-// either be a string, an integer, or implement encoding.TextUnmarshaler.
+// either be any string type, an integer, implement json.Unmarshaler, or
+// implement encoding.TextUnmarshaler.
 //
 // If a JSON value is not appropriate for a given target type,
 // or if a JSON number overflows the target type, Unmarshal