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).
I was converting the existing comment into a doxygen group.  Line comments (I believe you are talking about // comments) do not work for that.  Doxygen will just ignore the comment.  You'll get the group (because of the ```/* @{ */``` line) but it won't have a title.

I can do one of the following:

1 - Remove the grouping tags (the ```@name```, ```@{```, and ```@}``` stuff).  The resulting html document will still show all the members but it won't group them as "Basic item properties", etc
2 - Rewrite it as
```
/**
 * @name Basic item properties
 */
```
This feels a little too much for just the one line
3 - Rewrite it as
```
/**
 * @name Basic item properties
 * @{
 */
```
I prefer the look of ```/*@{*/``` as a single line for the beginning of groups over this.