doc: various minor improvements and fixes

* doc/figs/example.dot, doc/figs/example.y: New.
* doc/bison.texi: Prefer "token" to TOKEN.
Use @group where appropriate.
Adjust with style changes in the output (State 0, not state 0).
Fix some @ref that were missing the third argument.
Fix some incorrect line numbers.
Use "nonterminal", not "non-terminal".
Fix overfull and underfull TeX hboxes.
Put the comments in the index.
Remove duplicate index entries.
Fuse glossary entries where appropriate.
(Understanding): Improve the continuity between sections.
Use example.dot to show the whole graph.
* doc/Makefile.am: Adjust.

doc/figs/example.y

@@ -0,0 +1,4 @@
+%%
+exp: a ";" | b ".";
+a: "0";
+b: "0";