doc/syntax.tex: Improve the syntax notation.

Rather than having a bunch of special rules, define the notational
extensions in terms of the parametrized nonterminals we have anyway for
declarators, and set that on a more formal footing.

I'm not quite sure how powerful this formalism is.  I believe it's
strictly more powerful than context-free grammars, but I haven't decided
whether it's Turing-equivalent.  It doesn't matter much here either way,
because the actual grammar is indeed context-free even if the formalism
is more expressive than that.