[DOCS] Add note on accessing defaults in FlexForm attributs (#3612)

Follow-up on TYPO3-Documentation/TYPO3CMS-Reference-TSconfig#391 to also document at the "other end" how to access flexform defaults.

Releases: main, 12.4, 11.5

Documentation/ApiOverview/FlexForms/Index.rst

@@ -475,6 +475,25 @@ The key `flexform` is followed by the field which holds the FlexForm data
  * :ref:`TypoScript: flexform <t3tsref:data-type-gettext-flexform>`
 
 
+.. index:: pair: FlexForms; Default value
+.. _default-flexforms-attribute:
+
+Providing default values for FlexForms attributes
+-------------------------------------------------
+
+When a new content element with an attached FlexForm is created, the 
+default values for each FlexForm attribute is fetched from the
+:xml:`<default>` XML attribute within the specification of each 
+FlexForm attribute. If that is missing, an empty value will be 
+shown in the backend (:ref:`FormEngine <FormEngine>`)
+fields.
+
+While you can use page TSconfig's :ref:`t3tsref:pageTsTcaDefaults` to 
+modify defaults of usual TCA-based attributes, this is not 
+possible on FlexForms. This is because the values are calculated 
+at an earlier step in the Core workflow, where FlexForm values 
+have not yet been extracted.
+
 .. index:: pair: FlexForms; Fluid
 .. _read-flexforms-fluid: