Misleading documentation for Watchable Providers

[stv0g]

The documentation current says:

The koanf.Provider interface has a Watch(cb) method that asks a provider to watch for changes and trigger the given callback that can live reload the configuration.

which is not true:

https://pkg.go.dev/github.com/knadh/koanf#Provider

I think the paragraph should be changed to something along the lines:

Some koanf.Provider implementations provide a Watch(cb) method that asks a provider to watch for changes and trigger the given callback that can live reload the configuration.

[stv0g]

I also realized that the env provider implements Watch() while not even supporting it.

I would say that it be nice to have a koanf.WatchableProvider interface which is only implemented by those providers which support it so that we can simply test for it rather than handling the error.

[knadh]

Ah yes. Watch() must've been a part of the Provider interface at some point, which is why there are Watch() stubs that return errors across providers. Cleaned this.

[stv0g]

Thanks! This looks much cleaner