There is not much documentation about the SDK... apart from what is already included with it.
Will Day has some technical comments over some of the functions of the SDK on his metamod web site (
http://www.metamod.org)
Other than that, some bots have a very commented source code, you may want to have a look at my RACC template #2 for example. I always try to put 5 or 10 lines of explanations before writing the body of a function - the same goes when it's functions from the gameDLL or engine interface.