I'm not saying to put spec before rationale; just saying
to put a bit more examples so that the readers can get clearer
ideas of how srfi-105 looks like before he goes to rationale section.
Exactly what you suggested.

I don't think you need to explain why the rationale is so long---
if you need such an explanation, probably the rational is actually
too long.  I don't think it's too long, though; it contains valuable
discussion.   It's just that the reader needs to see what srfi-105 is
before understanding "why not this, not that" rationale.