Mention that env needs feature in dervie reference

[pksunkara]

https://github.com/clap-rs/clap/blob/v3.0.14/examples/derive_ref/README.md#arg-attributes

From: #3463

[epage]

There is a cost to each feature flag we create (e.g. code complexity; bloating of documentation, making it harder to find relevant information).

I wonder if we are getting the desired value out of having an env feature flag and if we should deprecate it.

[pksunkara]

I think we would probably need to decide how exactly we are going to make clap modular before deprecating this. We don't want to increase the churn by redoing this later.