add some docs about visit form javascript

docs/source/development.rst

@@ -176,6 +176,13 @@ The redirect to the stripe checkout server in Visit is done using a custom javas
 .. image:: _images/visit-form-redirect.png
   :alt: Visit form redirect instructions
 
+.. warning::
+  There is another method of adding javascript, namely in the form settings rather than the custom
+  page settings. These are the first settings you see when editing the form. Any javascript placed
+  here is activated for the whole form rather than a single page. Also, Javascript placed in the
+  form settings, overrides any javascript placed in page settings, so this will deactivate the
+  automatic redirect.
+
 Redirect URL
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