docs(toArray): added description and example for toArray (#4775)

Author: waseemahmad31

src/internal/operators/toArray.ts

@@ -9,6 +9,37 @@ function toArrayReducer<T>(arr: T[], item: T, index: number) {
   return arr;
 }
 
+/**
+ * Collects all source emissions and emits them as an array when the source completes.
+ *
+ * <span class="informal">Get all values inside an array when the source completes</span>
+ *
+ * ![](toArray.png)
+ *
+ * `toArray` will wait until the source Observable completes before emitting
+ * the array containing all emissions. When the source Observable errors no
+ * array will be emitted.
+ *
+ *  ## Example
+ * ```ts
+ * import { interval } from 'rxjs';
+ * import { toArray, take } from 'rxjs/operators';
+ *
+ * const source = interval(1000);
+ * const example = source.pipe(
+ *   take(10),
+ *   toArray()
+ * );
+ *
+ * const subscribe = example.subscribe(val => console.log(val));
+ *
+ * // output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+ *
+ * ```
+* @return An array from an observable sequence.
+* @method toArray
+* @owner Observable
+*/
 export function toArray<T>(): OperatorFunction<T, T[]> {
   return reduce(toArrayReducer, [] as T[]);
 }