gdb/doc: shorten some source lines, and prevent some line breaks

Building on the previous commit, this makes use of a trailing @ to
split long @deffn lines in the guile.texi source file.  This splitting
doesn't change how the document is laid out by texinfo.

I have also wrapped keyword and argument name pairs in @w{...} to
prevent line breaks appearing between the two.  I've currently only
done this for the longer @deffn lines, where a line break is
possible.  This makes the @deffn lines much nicer to read in the
generated pdf.