ENBSeries documentation different for each version of the mod, because not all parameters are the same, this depends from game

TES 5 Skyrim

Incomplete, but i'm working on it, grab it here.

Deus Ex 3 Human Revolution

No docs of ENBSeries for this game yet.

GTA 4 documentation

I don't have my own docs for GTA 4 version.
Documentation made by user from Poland can be found here.

ENBSeries old versions

Previous documentation of the mod with old enbseries.ini file format (like for GTA San Andreas) can be found here