bip32: doc fixup
Deriving child `XPub`s from a parent `XPub` was implemented in
//! as wrapped by the [`secp256k1` Rust crate].
//!
//! ## Limitations and further work
-//! - Deriving non-hardened [`XPub`]s from a parent [`XPub`] unsupported
//! - Only 24-word BIP39 mnemonics are supported
//! - BIP43, BIP44, BIP49, BIP84 not yet properly supported
//!
