style: use 'nonterminal' consistently

* doc/bison.texi: Formatting changes.
* src/gram.h, src/gram.c (nvars): Rename as...
(nnterms): this.
Adjust dependencies.
(section): New.  Use it.
Replace "non terminal" and "non-terminal" by "nonterminal".

src/lr0.c

@@ -86,7 +86,7 @@ state_list_append (symbol_number sym, size_t core_size, item_index *core)
   return res;
 }
 
-/* Symbols that can be "shifted" (including non terminals) from the
+/* Symbols that can be "shifted" (including nonterminals) from the
    current state.  */
 bitset shift_symbol;