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

[Tom Tromey]

>>>>> "Phil" == Phil Muldoon <pmuldoon@redhat.com> writes:

Phil> +  GDB 7.0 will include support for writing pretty-printers in Python.
Phil> +  A library of printers for STL classes already exists within the
Phil> +  libstdc++ svn repository.  For information on enabling these printers,
Phil> +  and for other GDB STL support options: please see
Phil> + <ulink url="http://sourceware.org/gdb/wiki/STLSupport"> "GDB Support
Phil> +  for STL" </ulink> in the GDB wiki.

I think it would be preferable to put the actual instructions into the
libstdc++ documentation.

Tom