reflect: clarify Value.Equal behavior for non-comparable values

reflect: clarify Value.Equal behavior for non-comparable values

The current implementation always returns false for non-comparable
values, update the doc to reflect that.

Change-Id: I87f2da408874b0a209c8f51949e3310da15c5904
Reviewed-on: https://go-review.googlesource.com/c/go/+/426195
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: xie cui <523516579@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: hopehook <hopehook@golangcn.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>