runtime/cgo: clarify Handle documentation
runtime/cgo: clarify Handle documentation

Fixes #45427

Change-Id: Ic67630a5f39d8789a4a30c6b4ee30946bc50382e
Reviewed-on: https://go-review.googlesource.com/c/go/+/308230
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Ainar Garipov <8000b44fa86eaec0eedca699a849fef3186f9f49@gmail.com>
Reviewed-by: Changkun Ou <7e15748cb48ac6c6b8fb0ddf350b06d1f53bbb2e@gmail.com>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

cloudflare/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go