go/ast: improve comment on FuncDecl.Body

"Forward declaration" suggests that declarations must precede calls.

Change-Id: I1b3a26b58f52a39bc11d75696df928a6b66c7313
Reviewed-on: https://go-review.googlesource.com/47073Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/ast/ast.go

@@ -927,7 +927,7 @@ type (
 		Recv *FieldList    // receiver (methods); or nil (functions)
 		Name *Ident        // function/method name
 		Type *FuncType     // function signature: parameters, results, and position of "func" keyword
-		Body *BlockStmt    // function body; or nil (forward declaration)
+		Body *BlockStmt    // function body; or nil for external (non-Go) function
 	}
 )