gdb/dwarf: update comment of cutu_reader::cutu_reader (the DWO variant)

The comment on this constructor is really outdated.  Update it to better
reflect the reality today.

I'd eventually like to change this cutu_reader constructor not to use
dwarf2_per_cu, because it seems like an abuse of dwarf2_per_cu just to
pass 3 values.  But for now, just document the existing behavior.

Change-Id: Id96db020c361e64d9b0d2f25d51950b206658aa2
Approved-By: Tom Tromey <tom@tromey.com>