docs: improve documentation on how to pass a stringified parameter list to HttpClient (#41010)

Fixes #40618

PR Close #41010

packages/common/http/src/client.ts

@@ -74,6 +74,14 @@ function addBody<T>(
  *    return this.httpClient.request('GET', this.heroesUrl, {responseType:'json', params});
  * }
  * ```
+ *
+ * Alternatively, the parameter string can be used without invoking HttpParams
+ * by directly joining to the URL.
+ * ```
+ * this.httpClient.request('GET', this.heroesUrl + '?' + 'name=term', {responseType:'json'});
+ * ```
+ *
+ *
  * ### JSONP Example
  * ```
  * requestJsonp(url, callback = 'callback') {
@@ -92,6 +100,7 @@ function addBody<T>(
  * ```
  *
  * @see [HTTP Guide](guide/http)
+ * @see [HTTP Request](api/common/http/HttpRequest)
  *
  * @publicApi
  */

packages/common/http/src/request.ts

@@ -119,6 +119,13 @@ export class HttpRequest<T> {
 
   /**
    * Outgoing URL parameters.
+   *
+   * To pass a string representation of HTTP parameters in the URL-query-string format,
+   * the `HttpParamsOptions`' `fromString` may be used. For example:
+   *
+   * ```
+   * new HttpParams({fromString: 'angular=awesome'})
+   * ```
    */
   // TODO(issue/24571): remove '!'.
   readonly params!: HttpParams;