Rewrite the client classification documentation in Kea ARM
Client classification is a complex topic. We started simple but over the years we have added more and more feature to client classification. While reviewing the most recent addition to client classification in #1139 (closed) we found that certain paragraphs are unclear. For example, see this thread: !686 (comment 115873). We think it may be now good time to rewrite the documentation about the client classification and perhaps add some diagrams explaining how it all works together.