Describe the problem as a bug, since it is not implied by the rest of
the pointer passing rules, and it may be possible to fix it.

Updates #19928

Change-Id: I2d336e7336b2a215c0b8cf909a203201ef1b054e
Reviewed-on: https://go-review.googlesource.com/123658Reviewed-by: Austin Clements <austin@google.com>