ENH: docs: Add documentation about flattening a PDF form

Author: PJBrs

docs/user/forms.md

@@ -36,6 +36,7 @@ writer.update_page_form_field_values(
     writer.pages[0],
     {"fieldname": "some filled in text"},
     auto_regenerate=False,
+    flatten=False,
 )
 
 writer.write("out-filled-form.pdf")
@@ -46,6 +47,12 @@ parameter is `True` by default for legacy compatibility, but this flags the PDF
 processor to recompute the field's rendering, and may trigger a "save changes"
 dialog for users who open the generated PDF.
 
+If you want to flatten your form, that is, keeping all form field contents while
+removing the form fields themselves, you can set `flatten=True` to convert form
+field contents to regular PDF content, and then use
+`writer.remove_annotations(subtypes="/Widget")` to remove all form fields. This
+will result in a flattened PDF.
+
 ## Some notes about form fields and annotations
 
 PDF forms have a dual-nature approach to the fields: