[RFA/doco] GDB Manual edition number and ISBN updates

[Joel Brobecker]

> Yes, of course.  I trust that you verified the ISBN.

Thanks! I verified the ISBN to the point where I am quite certain
that it is identical to the one that was given to me by the FSF.
However, I don't know of any other way to verify the ISBN itself.
There's isbnsearch.org which seems to have found that ISBN, but
there's no information associated, so if there is another way,
please let me know.

I did send a copy of the resulting PDF to the FSF as well, and I am
waiting for their OK.

-- 
Joel