OAK-1995: Improved SegmentNodeStore documentation

Add a warning about the complex internals of MemoryNodeBuilder

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1614818 13f79535-47bb-0310-9956-ffa450edef68
OAK-1995: Improved SegmentNodeStore documentation

Add some TODOs for areas that could do with extra documentation

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1614819 13f79535-47bb-0310-9956-ffa450edef68
OAK-1995: Improved SegmentNodeStore documentation

Document the magic constants used in MapRecord

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1614820 13f79535-47bb-0310-9956-ffa450edef68