cmd/cgo: add note about bug writing C pointers to uninitialized C memory
cmd/cgo: add note about bug writing C pointers to uninitialized C memory

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/123658
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

cloudflare/go,akutz/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,akutz/go,josharian/go,nawawi/go,akutz/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,akutz/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go