minor symfony#2432 [Doc] Mention how to work with Twig macros in Twig Components (javiereguiluz)

This PR was squashed before being merged into the 2.x branch.

Discussion
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[Doc] Mention how to work with Twig macros in Twig Components

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Issues        | -
| License       | MIT

Made during the #SymfonyHackday

Commits
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45f825e [Doc] Mention how to work with Twig macros in Twig Components

Author: Kocal

src/TwigComponent/doc/index.rst

@@ -365,6 +365,21 @@ You can even give the block default content. See
 :ref:`Passing HTML to Components via Block <embedded-components>`
 for more info.
 
+The only limitation when defining contents inside a component using the HTML syntax
+is that you cannot import macros using the ``_self`` keyword. You must always use
+the full template path:
+
+.. code-block:: html+twig
+
+    <twig:Alert>
+        {# ❌ this won't work #}
+        {% from _self import message_formatter %}
+        {# ✅ this works as expected #}
+        {% from 'some/path/template.html.twig' import message_formatter %}
+
+        {{ message_formatter('...') }}
+    </twig:Alert>
+
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