encoding/json: clarify unmarshaling behaviour on bad fields

Fixes #19526

Change-Id: Ifaaf454e0e89fdf4309118c2e2e6ac0d0a43c39d
Reviewed-on: https://go-review.googlesource.com/44711Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/decode.go

@@ -79,7 +79,9 @@ import (
 // or if a JSON number overflows the target type, Unmarshal
 // skips that field and completes the unmarshaling as best it can.
 // If no more serious errors are encountered, Unmarshal returns
-// an UnmarshalTypeError describing the earliest such error.
+// an UnmarshalTypeError describing the earliest such error. In any
+// case, it's not guaranteed that all the remaining fields following
+// the problematic one will be unmarshaled into the target object.
 //
 // The JSON null value unmarshals into an interface, map, pointer, or slice
 // by setting that Go value to nil. Because null is often used in JSON to mean