Docs: Fix incorrect size limitation in high-level hash module.
//!
//! # Panics:
//! A panic will occur if:
-//! - More than 2*(2^64-1) __bits__ of data are hashed.
+//! - More than 2*(2^64-1) bytes of data are hashed.
//!
//! # Security:
//! - This interface does not support supplying BLAKE2b with a secret key, and
