runtime/cgo: clarify that C code must not retain pointer

For #71566

Change-Id: I6dc365dd799d7b506b4a55895f1736d3dfd4684b
Reviewed-on: https://go-review.googlesource.com/c/go/+/647095
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/runtime/cgo/handle.go

@@ -61,10 +61,16 @@ import (
 //	}
 //
 // Some C functions accept a void* argument that points to an arbitrary
-// data value supplied by the caller. It is not safe to coerce a [cgo.Handle]
+// data value supplied by the caller. It is not safe to coerce a Handle
 // (an integer) to a Go [unsafe.Pointer], but instead we can pass the address
 // of the cgo.Handle to the void* parameter, as in this variant of the
-// previous example:
+// previous example.
+//
+// Note that, as described in the [cmd/cgo] documentation,
+// the C code must not keep a copy of the Go pointer that it receives,
+// unless the memory is explicitly pinned using [runtime.Pinner].
+// This example is OK because the C function myprint does not keep
+// a copy of the pointer.
 //
 //	package main
 //