docs(ajax): Adding more examples and cases of how ajax operator can be used.

src/internal/observable/dom/ajax.ts

@@ -5,15 +5,78 @@ import {  AjaxObservable, AjaxCreationMethod  } from './AjaxObservable';
  * It creates an observable for an Ajax request with either a request object with
  * url, headers, etc or a string for a URL.
  *
+ *
+ * ## Using ajax() to fetch the response object that is being returned from API.
+ * ```ts
+ * import { ajax } from 'rxjs/ajax';
+ * import { map, catchError } from 'rxjs/operators';
+ * import { of } from 'rxjs';
+ *
+ * const obs$ = ajax(`https://api.github.com/users?per_page=5`).pipe(
+ *   map(userResponse => console.log('users: ', userResponse)),
+ *   catchError(error => {
+ *     console.log('error: ', error);
+ *     return of(error);
+ *   })
+ * );
+ *
+ * ```
+ *
  * ## Using ajax.getJSON() to fetch data from API.
- * ```javascript
+ * ```ts
  * import { ajax } from 'rxjs/ajax';
  * import { map, catchError } from 'rxjs/operators';
+ * import { of } from 'rxjs';
  *
  * const obs$ = ajax.getJSON(`https://api.github.com/users?per_page=5`).pipe(
  *   map(userResponse => console.log('users: ', userResponse)),
- *   catchError(error => console.log('error: ', error))
+ *   catchError(error => {
+ *     console.log('error: ', error);
+ *     return of(error);
+ *   })
+ * );
+ *
+ * ```
+ *
+ * ## Using ajax() with object as argument and method POST with a two seconds delay.
+ * ```ts
+ * import { ajax } from 'rxjs/ajax';
+ * import { of } from 'rxjs';
+ *
+ * const users = ajax({
+ *   url: 'https://httpbin.org/delay/2',
+ *   method: 'POST',
+ *   headers: {
+ *     'Content-Type': 'application/json',
+ *     'rxjs-custom-header': 'Rxjs'
+ *   },
+ *   body: {
+ *     rxjs: 'Hello World!'
+ *   }
+ * }).pipe(
+ *   map(response => console.log('response: ', response)),
+ *   catchError(error => {
+ *     console.log('error: ', error);
+ *     return of(error);
+ *   })
+ * );
+ *
+ * ```
+ *
+ * ## Using ajax() to fetch. An error object that is being returned from the request.
+ * ```ts
+ * import { ajax } from 'rxjs/ajax';
+ * import { map, catchError } from 'rxjs/operators';
+ * import { of } from 'rxjs';
+ *
+ * const obs$ = ajax(`https://api.github.com/404`).pipe(
+ *   map(userResponse => console.log('users: ', userResponse)),
+ *   catchError(error => {
+ *     console.log('error: ', error);
+ *     return of(error);
+ *   })
  * );
+ *
  * ```
  */
 export const ajax: AjaxCreationMethod = AjaxObservable.create;