fixup! docs: Document the Cipher struct

src/cipher/mod.rs

@@ -128,7 +128,9 @@ impl Cipher {
         Self { keys }
     }
 
-    /// Creates a new [`Cipher`] from the given 32-byte raw key.
+    /// Creates a new [`Cipher`] from the given raw key. The key is expected to
+    /// be 32 bytes in length, but we expect an unsized slice for
+    /// compatibility with the libolm API.
     ///
     /// The key is deterministically expanded into a 32-byte AES key, a 32-byte
     /// MAC key, and a 16-byte initialization vector (IV) using HKDF, with