doc: Distribute a modern version of texinfo.tex with the docs

* doc/texinfo.tex: Ship a more recent texinfo.tex, from texinfo 6.1, so we
  don't have to worry about that.  E.g., this should allow UTF-8 unicode
  chars to be used directly in the text.
* doc/Makefile.am: Add previous to EXTRA_DIST
3 files changed