[AssetMapper] Adding info about deleting compiled assets

Page: https://symfony.com/doc/6.4/frontend/asset_mapper.html#serving-assets-in-dev-vs-prod

First: I'm not sure if this is really true for any (local server) configuration - I took the info from `asset-map:compile`s warning message (which I'm trying to improve in symfony/symfony#54106 BTW)

The message I'm trying to get across: I've you're doing this on your dev machine (as part of the deploy process), you should set up a process that deletes those files afterwards!
Maybe this should be repeated at https://symfony.com/doc/6.4/frontend/asset_mapper.html#deploying-with-the-assetmapper-component again?
Or should those two heading even be merged?

Author: ThomasLandauer

frontend/asset_mapper.rst

@@ -81,7 +81,7 @@ Serving Assets in dev vs prod
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 In the ``dev`` environment, the URL ``/assets/images/duck-3c16d9220694c0e56d8648f25e6035e9.png``
-is handled and returned by your Symfony app.
+is handled and returned by your Symfony app - but only if ``public/assets/`` is empty (see below).
 
 For the ``prod`` environment, before deploy, you should run:
 
@@ -93,6 +93,9 @@ This will physically copy all the files from your mapped directories to
 ``public/assets/`` so that they're served directly by your web server.
 See :ref:`Deployment <asset-mapper-deployment>` for more details.
 
+If you compiled your assets on your development machine, you need to delete them again,
+in order to make Symfony serve the current versions from ``assets/`` again.
+
 .. tip::
 
     If you need to copy the compiled assets to a different location (e.g. upload