[DOC beta] remove needless prohibition

Originally introduced here:
https://github.com/emberjs/ember.js/pull/4403/files
Discussion question: https://discuss.emberjs.com/t/why-should-i-avoid-transitionto-and-full-urls/14870
This prohibition does not seem necessary so we should remove to avoid
teaching overhead.

Author: eflanagan0

packages/@ember/-internals/routing/lib/ext/controller.ts

@@ -113,8 +113,7 @@ ControllerMixin.reopen({
     ```
 
     It is also possible to pass a URL (a string that starts with a
-    `/`). This is intended for testing and debugging purposes and
-    should rarely be used in production code.
+    `/`).
 
     ```javascript
     aController.transitionToRoute('/');
@@ -195,8 +194,7 @@ ControllerMixin.reopen({
     ```
 
     It is also possible to pass a URL (a string that starts with a
-    `/`). This is intended for testing and debugging purposes and
-    should rarely be used in production code.
+    `/`).
 
     ```javascript
     aController.replaceRoute('/');

packages/@ember/-internals/routing/lib/system/route.ts

@@ -628,8 +628,7 @@ class Route extends EmberObject implements IRoute {
     ```
 
     It is also possible to pass a URL (a string that starts with a
-    `/`). This is intended for testing and debugging purposes and
-    should rarely be used in production code.
+    `/`).
 
     ```javascript
     this.transitionTo('/');