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

Part of #8511.

modules/@angular/platform-browser/src/dom/dom_adapter.ts

@@ -29,6 +29,9 @@ export function setRootDomAdapter(adapter: DomAdapter) {
 /* tslint:disable:requireParameterType */
 /**
  * Provides DOM operations in an environment-agnostic way.
+ *
+ * @security Tread carefully! Interacting with the DOM directly is dangerous and
+ * can introduce XSS risks.
  */
 export abstract class DomAdapter {
   public resourceLoaderType: Type<any> = null;