DOC: Notes about update_page_form_field_values(auto_regenerate) (#2359)

Co-authored-by: dmjohnsson23 <dmjohn235@gmail.com>

Author: MartinThoma

docs/user/forms.md

@@ -27,10 +27,17 @@ fields = reader.get_fields()
 writer.append(reader)
 
 writer.update_page_form_field_values(
-    writer.pages[0], {"fieldname": "some filled in text"}
+    writer.pages[0],
+    {"fieldname": "some filled in text"},
+    auto_regenerate=False,
 )
 
 # write "output" to pypdf-output.pdf
 with open("filled-out.pdf", "wb") as output_stream:
     writer.write(output_stream)
 ```
+
+Generally speaking, you will always want to use `auto_regenerate=False`. The
+parameter is `True` by default for legacy compatibility, but this flags the PDF
+Viewer to recompute the field's rendering, and may trigger a "save changes"
+dialog for users who open the generated PDF.