Update deferrable.md

[peterthomassen]

Without this change, people reading just the examples will think that URIs must be given to pgtrigger.constraints, whereas they are optional.

I got also quite confused that the URIs aren't passed as a list, but *uris is unzipped by the receiving function. I find that a little unintuitive.

Another suggestion is to skip over non-deferrable triggers, at least when no URIs are given.

[wesleykendall]

The initial reason for *uris is that the vast majority of pgtrigger.ignore()/pgtrigger.constraints() invocations are over just one trigger. I can definitely support both list/*uris syntax though.

You are correct that the behavior of pgtrigger.constraints() is annoying. It should definitely skip non-deferrable triggers when none are supplied.

For now I will merge just this change in the docs. I'm considering changing the behavior of constraints(), ignore(), etc, to require passing in trigger URIs since some people have been bitten by this already. Will open a discussion

thanks for your change, will merge soon