Update the contextual difficulty module doc
//! Block difficulty adjustment calculations for contextual validation.
+//!
+//! This module supports the following consensus rule calculations:
+//! * `ThresholdBits` from the Zcash Specification,
+//! * the Testnet minimum difficulty adjustment from ZIPs 205 and 208, and
+//! * `median-time-past`.
use chrono::{DateTime, Duration, Utc};
use primitive_types::U256;
