docs(router): add api docs

modules/@angular/router/src/url_tree.ts

@@ -63,7 +63,7 @@ function containsSegmentHelper(
 /**
  * A URL in the tree form.
  *
- * @experimental
+ * @stable
  */
 export class UrlTree {
   /**
@@ -76,6 +76,9 @@ export class UrlTree {
   toString(): string { return new DefaultUrlSerializer().serialize(this); }
 }
 
+/**
+ * @stable
+ */
 export class UrlSegment {
   /**
    * @internal
@@ -100,7 +103,7 @@ export class UrlSegment {
 
 
 /**
- * @experimental
+ * @stable
  */
 export class UrlPathWithParams {
   constructor(public path: string, public parameters: {[key: string]: string}) {}