update initial route documentation (#57136)

packages/flutter/lib/src/widgets/app.dart

@@ -404,10 +404,11 @@ class WidgetsApp extends StatefulWidget {
   /// Defaults to [Window.defaultRouteName], which may be overridden by the code
   /// that launched the application.
   ///
-  /// If the route contains slashes, then it is treated as a "deep link", and
-  /// before this route is pushed, the routes leading to this one are pushed
-  /// also. For example, if the route was `/a/b/c`, then the app would start
-  /// with the three routes `/a`, `/a/b`, and `/a/b/c` loaded, in that order.
+  /// If the route name starts with a slash and has multiple slashes in it, then
+  /// it is treated as a "deep link", and before this route is pushed, the
+  /// routes leading to this one are pushed also. For example, if the route was
+  /// `/a/b/c`, then the app would start with the three routes `/a`, `/a/b`, and
+  /// `/a/b/c` loaded, in that order.
   ///
   /// Intermediate routes aren't required to exist. In the example above, `/a`
   /// and `/a/b` could be skipped if they have no matching route. But `/a/b/c` is

packages/flutter/lib/src/widgets/navigator.dart

@@ -2151,9 +2151,10 @@ class Navigator extends StatefulWidget {
   /// This is the default value of [onGenerateInitialRoutes], which is used if
   /// [initialRoute] is not null.
   ///
-  /// If this string contains any `/` characters, then the string is split on
-  /// those characters and substrings from the start of the string up to each
-  /// such character are, in turn, used as routes to push.
+  /// If this string starts with a `/` character and has multiple `/` characters
+  /// in it, then the string is split on those characters and substrings from
+  /// the start of the string up to each such character are, in turn, used as
+  /// routes to push.
   ///
   /// For example, if the route `/stocks/HOOLI` was used as the [initialRoute],
   /// then the [Navigator] would push the following routes on startup: `/`,