Docs: Corrections to kdf use case description
//! Key derivation.
//!
//! # Use case:
-//! A Key Derivation Function (KDF) can be used if some given input key material (`ikm`)
+//! `orion::kdf` can be used if some given input key material (`ikm`)
//! needs to be converted into a specific length, i.e. "stretched", before it's suitable for use as
//! input to other cryptographic operations.
//!
//! An example of this could be deriving multiple keys using `orion::kdf`, with different `info` parameters for each key,
//! using a single starting key. These derived keys could then later be used
-//! as a secret keys for data encryption with `orion::aead`, password hashing with `orion::pwhash`, etc.
+//! as secret keys for data encryption with `orion::aead`, password hashing with `orion::pwhash`, etc.
//!
//! # About:
//! - Uses HKDF-HMAC-SHA512.
