minor #1538 [Doc] Add more examples of adding attributes in Live Components (javiereguiluz)

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

Discussion
----------

[Doc] Add more examples of adding attributes in Live Components

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

Note: I tried this in a Symfony app and it didn't work ... so I'm opening this PR to ask you if this should work or if `addAttribute()` is only for empty attributes that don't define a value.

Thanks!

Commits
-------

a6f2fe6 [Doc] Add more examples of adding attributes in Live Components

Author: weaverryan

src/LiveComponent/doc/index.rst

@@ -929,16 +929,22 @@ a class:
     <div data-loading="removeClass(opacity-50)">...</div>
 
     <!-- add multiple classes when loading -->
-    <div data-loading="addClass(opacity-50 disabled)">...</div>
+    <div data-loading="addClass(opacity-50 text-muted)">...</div>
 
-Sometimes you may want to add or remove an attribute when loading. That
+Sometimes you may want to add or remove an HTML attribute when loading. That
 can be accomplished with ``addAttribute`` or ``removeAttribute``:
 
 .. code-block:: html+twig
 
     <!-- add the "disabled" attribute when loading -->
     <div data-loading="addAttribute(disabled)">...</div>
 
+.. caution::
+
+    The ``addAttribute()`` and ``removeAttribute()`` functions only work with
+    empty HTML attributes (``disabled``, ``readonly``, ``required``, etc.) and
+    not with attributes that define their values (e.g. this won't work: ``addAttribute(style='color: red')``).
+
 You can also combine any number of directives by separating them with a
 space: