[libstdc++] Add brief documentation narrative for GDB pretty-printers. Add links to GDB wiki/manual

[Benjamin Kosnik]

> Thanks.   I don't have write after approval status, so could either
> you or Tom do it? Or if you want to me do it, wave the appropriate
> magic wand ;)

Done. 

If you want WAA for libstdc++ (given your CL entries for the
python work it might be a good idea) please fill out the webform. I'm
willing to approve.

-benjamin