How do you enable a Rust "crate feature"?

I'm trying to use rand::SmallRng. The documentation says

This PRNG is feature-gated: to use, you must enable the crate feature small_rng.

I've been searching and can't figure out how to enable "crate features". The phrase isn't even used anywhere in the Rust docs. This is the best I could come up with:

[features]
default = ["small_rng"]

But I get:

Feature default includes small_rng which is neither a dependency nor another feature

Are the docs wrong, or is there something I'm missing?

Specify the dependencies in Cargo.toml like so:

[dependencies]
rand = { version = "0.7.2", features = ["small_rng"] }

Alternatively:

[dependencies.rand]
version = "0.7.2"
features = ["small_rng"]

Both work.