[vtkusers] vtkCollection API documentation suggestion

[N Smethurst]

Since I just spent a while working this one out, I thought it would be wise 
to include a note in the documentation of this class, specifically in the 
IsItemPresent(vtkObject*) method.

If I am correct, vtkCollection uses 0-indexing. However, this method uses 
1-indexing! Hence using a position int obtained from:

int pos = IsItemPresent(vtkObject*);

later on with:

vtkObject* GetItemAsObject(pos);

needs a decrement:

vtkObject* GetItemAsObject(pos-1);

I thought it would be worth noting this in the API documentation since there 
is no mention of it (other than in the code itself of course).

Nic