docs(security): mark the various DomAdapters as unsafe. (#10868)

Part of #8511.

modules/@angular/platform-browser/src/browser/browser_adapter.ts

@@ -64,6 +64,9 @@ var _chromeNumKeyPadMap = {
 
 /**
  * A `DomAdapter` powered by full browser DOM APIs.
+ *
+ * @security Tread carefully! Interacting with the DOM directly is dangerous and
+ * can introduce XSS risks.
  */
 /* tslint:disable:requireParameterType */
 export class BrowserDomAdapter extends GenericBrowserDomAdapter {