Document usage without crate's cache file

[Enyium]

My GUI app uses its own JSON config file to keep track of whether an update check should be performed. According to my understanding of the following code snippet, calling update_informer::new(...).interval(Duration::ZERO) is the way to achieve this:

        // If the interval is zero, don't use the cache file
        let latest_version = if self.interval.is_zero() {
            match R::get_latest_version(client, &pkg)? {
                Some(v) => v,
                None => return Ok(None),
            }
        } else {

But this isn't documented. Could you please document this usage as a way to completely avoid that the crate does anything on the file system, if I understood this correctly?

BTW: The docs text on interval() says: "An interval in seconds." But it's a Duration, which isn't a second value per se.

[github-actions[bot]]

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[Enyium]

@mgrachev, please react.

[mgrachev]

@Enyium 👋 Thank you for your feedback!

Any PR to improve the documentation is welcome 😊