All API functions covered are listed in api.def. Detailed descriptions of those functions should be easy to find on the Microsoft website, but you need to look yourself - lite-C doesn't include documentation about them.

For adding API functions, see:

http://manual.conitec.net/litec_api.htm
http://manual.conitec.net/litec_c.htm

For learning how to add a very complex API, like DirectX, it's best to have a look into the include\d3d9.h file.