pico-8 comes with a manual.txt or something
the pico-8 Wikia is very detailed
the main website's BBS has the rest, as well as a ton of examples
Trasevol_dog's patreon and twitter are a great resource as well as his games on the BBS
https://neko250.github.io/pico8-api/
https://www.lexaloffle.com/bbs/?tid=28207