[DOCS] Mention TCAdefaults do not apply to flexforms (#391)

Through https://forge.typo3.org/issues/102239 I noticed that flexforms are an edge-case that could be clarified here. Open to feedback for changing the wording on it. :-)

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Co-authored-by: Chris Müller <2566282+brotkrueml@users.noreply.github.com>

Documentation/PageTsconfig/TcaDefaults.rst

@@ -33,3 +33,7 @@ Example:
 
    # Show newly created pages by default
    TCAdefaults.pages.hidden = 0
+
+..  note::
+    `TCAdefaults` is not applied to :ref:`FlexForm <t3coreapi:flexforms>` values. These can only be addressed via
+    :xml:`<default>` elements within the FlexForm data structure.