[Reference][Form Types] Add missing docs for "action" and "method" option

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #3410

I'm not sure of adding these two options as separate rst-files because I don't think they will get listed as `inherited` options. But no other options are embedded directly. Please let me know if I can improve that.