(1) In the Specification section, it says: "All of this syntax not
already defined by an implementation is exported by this library."

What does it concretely mean about the set of exports of the (srfi
204) library? The set of exported bindings should be stable and not
vary between one implementation and the other.

The obvious solution is that the (srfi 204) library (re-)exports all syntax.

This leads to the next question: Which of the syntax will be
guaranteedly bound using SRFI 206 when available?

(2) In the description of match-let and friends, the meta-variable
`var` is used. While there is no formal specification, I guess `var`
should be `pattern` as with the match form and match-lambda(*).

(3) Is the ellipsis only allowed at the end of a list (or vector) of
patterns? If so, why? Syntax-rules don't have this restriction.

(4) Match, match-let, and the other forms apparently cannot bind
identifiers whose current binding is the same as one of the following:
and, not, ..., or, =, and whatever.

This should be added to the specification and the list ("... and
whatever") be completed.