You could read the spec of your target OpenGL version at OpenGL.org
http://www.opengl.org/registry/
Say that you want to learn the non-deprecated functionality of OpenGL 3.3, you grab the OpenGL 3.3 core spec pdf. Though its documentation about "what it does" rather than a tutorial of "how to use it". Or grab the GLSL spec if you want to know all the things you can do inside your shaders.