The render layer section at `/render/post_process/layers.rst` is a nice technical reference for all the settings, but it also needs to explain how to use render layers, perhaps by and example or two.
NOTE: This page should not contain too much information that is specific to a certain render engine. Though this is of course unavoidable in a practical example, its purpose is to explain the general concept of render layers, not which buttons to press specifically to use them.