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 <adonovan@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
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