cmd/go: add comments to C.* special functions

cmd/go: add comments to C.* special functions

Adding comments to these functions help IDE tooling to display
meaningful documentation, p.e. on hover.

Tested with gopls and vscode.

Updates #51134

Change-Id: Ie956f7cf192af0e828def4a141783f3a2589f77d
Reviewed-on: https://go-review.googlesource.com/c/go/+/386414
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>