doc/syntax.tex: Refactor the <class-definition> syntax.

  * Attach the semicolon terminators to the individual <...-item>
    syntaxes rather than including them in <class-item>.

  * Conversely, pull the property sets out of the individual items and
    into the top-level syntax, on the grounds that all class items have
    property sets as a fundamental part of the high-level syntax.