cgo: update documentation on calling C variadic functions
cgo: update documentation on calling C variadic functions

The current implementation does not support calling C variadic
functions (as discussed in #975). Document that.

Fixes #23537

Change-Id: If4c684a3d135f3c2782a720374dc4c07ea66dcbb
Reviewed-on: https://go-review.googlesource.com/90415
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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