Document that ivy-read's action function does not necessarily take a string argument

[alphapapa]

The docstring and manual for ivy-read say that its action argument is a function that takes a string as its argument, e.g.:

ACTION is a function to call after selecting a candidate.
It takes the candidate, which is a string, as its only argument.

But if the collection is an alist, the action function receives the entire cons as its argument, not only the key string.

This is misleading, but it's also a minor barrier to entry. For example, I thought that I would have to do the assoc myself based on the string, while Ivy actually handles that itself.

[abo-abo]

Thanks.