Modifying the best practice to use form_start() instead of <form

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | no
| Applies to | 2.3+
| Fixed tickets | #4402 

Hi guys!

See #4402. This proposes recommending to use `form_start` for the reasons there. There are pros and cons, but after nice chat, I think the pros outweigh the cons.

/cc @webmozart 

Thanks!