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 {

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

@@ -14,6 +14,9 @@ import {isFunction, isPresent} from '../facade/lang';
 
 /**
  * Provides DOM operations in any browser environment.
+ *
+ * @security Tread carefully! Interacting with the DOM directly is dangerous and
+ * can introduce XSS risks.
  */
 export abstract class GenericBrowserDomAdapter extends DomAdapter {
   private _animationPrefix: string = null;

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;

modules/@angular/platform-server/src/parse5_adapter.ts

@@ -35,6 +35,12 @@ function _notImplemented(methodName: any /** TODO #9100 */) {
 }
 
 /* tslint:disable:requireParameterType */
+/**
+ * A `DomAdapter` powered by the `parse5` NodeJS module.
+ *
+ * @security Tread carefully! Interacting with the DOM directly is dangerous and
+ * can introduce XSS risks.
+ */
 export class Parse5DomAdapter extends DomAdapter {
   static makeCurrent() {
     parser = new parse5.Parser(parse5.TreeAdapters.htmlparser2);