Add explanation about Navigator to showDialog doc (#14666)

* add explanation about navigator to showDialog doc

* update doc by review

packages/flutter/lib/src/material/dialog.dart

@@ -467,6 +467,11 @@ class _DialogRoute<T> extends PopupRoute<T> {
 /// Returns a [Future] that resolves to the value (if any) that was passed to
 /// [Navigator.pop] when the dialog was closed.
 ///
+/// The dialog route created by this method is pushed to the root navigator.
+/// If the application has multiple [Navigator] objects, it may be necessary to
+/// call `Navigator.of(context, rootNavigator: true).pop(result)` to close the
+/// dialog rather just 'Navigator.pop(context, result)`.
+///
 /// See also:
 ///  * [AlertDialog], for dialogs that have a row of buttons below a body.
 ///  * [SimpleDialog], which handles the scrolling of the contents and does