Improve the exception from \Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber::wrapControllerExecutionInRenderContext by linking to an example of how to use executeInRenderContext

One reason why it's such an issue is that this rendering may be entirely hidden from developers building APIs without touching the render system.

One such case would be having to prepare a URL and needing it as a string for inclusion in a cacheable JSON response or a redirect. This goes through Url::toString(TRUE) (to obtain the cacheability metadata for the route) which will eventually invoke MetadataBubblingUrlGenerator::generateFromRoute, introducing the renderer for what appears to be a non-rendered response.
This is entirely non-obvious before digging into the execution trace, and mystifying even after finding it: without digging into the arcane of rendering context, there appears to be no reason to even touch the renderer just to get a URL from a route.