[ABNF] Refine the rule for input titles.

Since we only allow four kinds of input section titles, corresponding to the
public/private/constant/const characterization of function inputs, it seems
beneficial to put this into the grammar, where it is easily captured.

Note that the previous version of the rule, which uses `identifier`, is not
quite right, because, for example, `public` is not an identiifer (it is a
keyword). So the rule would have to be modified anyways.

docs/grammar/abnf-grammar.txt

@@ -369,7 +369,7 @@ input-expression = literal
 
 input-item = identifier ":" input-type "=" input-expression ";"
 
-input-title = "[" identifier "]"
+input-title = "[" ( %s"public" / %s"private" / %s"constant" / %s"const" ) "]"
 
 input-section = input-title *input-item