1 - In avisynth folder, there is another, docs and inside this one, english. It contain html files, search for index and create a direct access where you want. From this file you can navigate and read about all internal avisynth filters.
2 - You can read about that, online, here:
http://avisynth.org/index.php?page=AviSynthManual
Also using site search engine, if you want any specific information.
3 - If you want any external filter info, visite
http://www.avisynth.org/warpenterprises/ and follow the links that comes at end of filters lines.
For example,
http://www.indeus.com/sansgrip/avisynth/