docs: Mention that the custom types in the crypto crate implement zer…

…oize

crates/matrix-sdk-crypto/src/types/mod.rs

@@ -14,14 +14,16 @@
 
 //! Module containing customized types modeling Matrix keys and events.
 //!
-//! These types were mostly taken from the Ruma project. The types differ in two
-//! important ways to the Ruma types of the same name:
+//! These types were mostly taken from the Ruma project. The types differ in a
+//! couple of important ways to the Ruma types of the same name:
 //!
 //! 1. They are using vodozemac types so we directly deserialize into a
 //!    vodozemac Curve25519 or Ed25519 key.
 //! 2. They support lossless serialization cycles in a canonical JSON supported
 //!    way, meaning the white-space and field order won't be preserved but the
 //!    data will.
+//! 3. Types containing secrets implement the [`Zeroize`] and [`ZeroizeOnDrop`]
+//!    traits to clear out any memory containing secret key material.
 
 use std::{
     borrow::Borrow,