This is just a collection of existing API Documentation that I am slowly adding to the specification:

https://github.com/Arcanemagus/plex-api/wiki/Plex-Web-API-Overview  
https://github.com/Arcanemagus/plex-api/wiki    
https://github.com/jc21/plex-api/blob/master/docs/Filter.md     
https://www.plexopedia.com/plex-media-server/api/   
https://www.plexopedia.com/plex-media-server/api/filter/