Documentation has been a bit of a challenge in my experience. There are some high-level docs at https://trac.webkit.org/wiki though many are 10-15 years old at this point. My approach has been to look at the commit history for the file to see if the changesets shed any light, and sometimes go to the attached bugzilla link to see if there was any discussion about the change there. Then attach a debugger and step through to try to uncover how the classes relate to one another.