Docs: incorrect description of parameter callback signature

[sphuber]

The section still claims the callback takes only two arguments where it has been three for a while now.

[j-holt]

I've never actually used this feature of Click before. I guess I've just never needed it, but this seemed like an easy first issue to tackle. I played around with the feature a bit in my development environment before creating the PR for the docs.

You're exactly right! If you write the callback function to accept only two arguments, then your app won't run. It raises a TypeError because the callback function was given too many arguments.

I'm assuming that this was a just a minor oversight by the last person editing the docs because the example code works just fine. (Edit: It looks like this piece of the doc just wasn't updated when the signature changed in version 2.0 in 2014).

[sphuber]

@jholtio exactly. I already opened #2114 to address the problem. Just opened the issue first because that is the policy before opening a PR.