cmd/cgo: improve documentation

Add a link to runtime/cgo.Handle in documentation.

Link: https://pkg.go.dev/runtime/cgo#Handle

Author: ariel-anieli

src/cmd/cgo/doc.go

@@ -425,8 +425,8 @@ and of course there is nothing stopping the C code from doing anything
 it likes. However, programs that break these rules are likely to fail
 in unexpected and unpredictable ways.
 
-The runtime/cgo.Handle type can be used to safely pass Go values
-between Go and C. See the runtime/cgo package documentation for details.
+The type [runtime/cgo.Handle] can be used to safely pass Go values
+between Go and C.
 
 Note: the current implementation has a bug. While Go code is permitted
 to write nil or a C pointer (but not a Go pointer) to C memory, the