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/reader.c

@@ -818,7 +818,7 @@ check_and_convert_grammar (void)
   }
 
   aver (nsyms <= SYMBOL_NUMBER_MAXIMUM);
-  aver (nsyms == ntokens + nvars);
+  aver (nsyms == ntokens + nnterms);
 
   /* Assign the symbols their symbol numbers.  */
   symbols_pack ();