Add doxygen comments to itemdef.h

I've added doxygen comments to itemdef.h.  I also removed a few function declarations from IWritableItemDefManager since they are already declared in its parent (IItemDefManager).
Having this as @CItemDefManager is weird but I guess it's better than splitting the comments between IItemDefManager and IWritableItemDefManager. Those two should have a short comment that only says "go look in @CItemDefManager" so that it's easy to find where the documentation is.