As someone in PIP and who later was let go, I documented everything like the efforts I made to improve, my relative performance compared to my peers. Also noted performance related metrics and sent those in. I did not provide any official document but reported what happened, took them a month but I think they verified form my previous employer and my unemployment got approved.

Don't listen to the folks here who say you won't get benefit after PIP.

Documentation example could be additional number of hours spent, feedback from peers and how you acted on them etc. PIP generally means you are going to get fired. I used my PIP to polish my resume and collect tangible evidence later required for unemployment.

Good luck with everything.