[AssetMapper]: Adding info that always *all* entrypoints are included in the importmap

Page: https://symfony.com/doc/6.4/frontend/asset_mapper.html#page-specific-css-javascript

Info is taken from symfony/symfony#54377 (comment)

Author: ThomasLandauer

frontend/asset_mapper.rst

@@ -1081,8 +1081,10 @@ loads the ``app.js`` file *and* the ``checkout.js`` file. It's important
 to *not* call ``parent()`` in the ``importmap`` block. Each page can only
 have *one* importmap, so ``importmap()`` must be called exactly once.
 
-If, for some reason, you want to execute *only* ``checkout.js``
+If you want to execute *only* ``checkout.js``
 and *not* ``app.js``, pass only ``checkout`` to ``importmap()``.
+In this case, still **both** files are added to ``<script type="importmap">``,
+but only ``checkout.js`` is executed (via ``<link rel="modulepreload">``).
 
 Using a Content Security Policy (CSP)
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