Make the documentation more explicit that it is not safe to directly
compare Value. Get straight to the point on how to do it correctly.

Updates #18871

Change-Id: I2aa3253f779636b2f72a1aae8c9bb45d3c32c902
Reviewed-on: https://go-review.googlesource.com/36018Reviewed-by: Keith Randall <khr@golang.org>