cmd/cgo: document that structs and arrays don't work with //export

Fixes #18412

Change-Id: Ib457eeced7820517aa35df9e7dfda1c0de4ac004
Reviewed-on: https://go-review.googlesource.com/52852
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cgo/doc.go

@@ -241,7 +241,10 @@ They will be available in the C code as:
 found in the _cgo_export.h generated header, after any preambles
 copied from the cgo input files. Functions with multiple
 return values are mapped to functions returning a struct.
+
 Not all Go types can be mapped to C types in a useful way.
+Go struct types are not supported; use a C struct type.
+Go array types are not supported; use a C pointer.
 
 Using //export in a file places a restriction on the preamble:
 since it is copied into two different C output files, it must not

src/cmd/cgo/out.go

@@ -1223,8 +1223,9 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 			// Slice: pointer, len, cap.
 			return &Type{Size: p.PtrSize * 3, Align: p.PtrSize, C: c("GoSlice")}
 		}
+		// Non-slice array types are not supported.
 	case *ast.StructType:
-		// TODO
+		// Not supported.
 	case *ast.FuncType:
 		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: c("void*")}
 	case *ast.InterfaceType: