Yes, many toolkits got documentation these days! The documentation can be in several variants, from the hard-core developer-variant where all documentation is put directly into the source code itself, to the full fledged Help System or database. The first variant is not necessary easy to get at!
The Notes APIs comes with a separate Notes Help Database describing the APIs in detail. Be aware that these files often are a separate download!
My recommendation is to take a look at the documentation. Very often it describes how the toolkits is constructed and what you'll find in the toolkit. It's often the blueprint to the toolkit.
Generally the documentation are put into the directory doc.
E:\Toolkits\LibraryX\doc
Look out for files like .doc (Word), .hlp (old WinHelp format) or .chm (new HTMLHelp format). Even look for .nsf (Notes databases!!!) :-)