Doc: Document new double finalization error check
//!
//! See [RFC](https://tools.ietf.org/html/rfc2104#section-2) for more information.
//!
+//! # Exceptions:
+//! An exception will be thrown if:
+//! - Either `finalize()` or `finalize_with_dst()` is called twice in a row without calling `reset()`
+//! in between
+//!
//! # Security:
//! The secret key should always be generated using a CSPRNG. The `gen_rand_key` function
//! in `util` can be used for this. The recommended length for a secret key is the SHA functions digest
