[boost] Add conversion op doc to scoped_ptr

This may already be known / in the queue / in CVS, but in case not ... The scoped_ptr synopsis and members does not document the unspecified-bool-type conversion operator that scoped_ptr provides (I didn't check scoped_array to see if the same issue is present or not.) I found this when a couple members of my team converted code from shared_ptr to scoped_ptr, with the original code relying on the conversion operator, and I was surprised when everything compiled cleanly (without adding the appropriate get() calls). Cliff