Warn about using form fields in Fluid components

Author: linawolf

Documentation/Global/Form/Index.rst

@@ -113,6 +113,23 @@ where (optional and configurable) validation occurs.
 Only valid data is passed on to the action and stored in
 the database.
 
+..  _typo3-fluid-form-components:
+
+Using f:form.<somefield> in Fluid components
+============================================
+
+..  warning::
+
+    Form input fields like the `Form.textfield ViewHelper <f:form.textfield> <https://docs.typo3.org/permalink/t3viewhelper:typo3-fluid-form-textfield>`_
+    need to find their parent form `<f:form>` within the rendering context.
+
+    They cannot be used in Fluid Components in TYPO3 13.4.
+
+In `Components <https://docs.typo3.org/permalink/fluid:components>`_ 
+(introduced with Fluid 4.3) a new rendering context is created for 
+each Fluid component. Therefore form fields loose the connection
+to their parent form and cannot be used.
+
 ..  _typo3-fluid-form-arguments:
 
 Arguments of the form ViewHelper