Add a comment showing how to update a hash generically.

This isn't obvious from the docs otherwise.

Change-Id: Id48a2cc18762a779e4834b6f6c1bea33020f6757
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/69807
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: Adam Langley <agl@google.com>

rust/bssl-crypto/src/digest.rs

diff --git a/rust/bssl-crypto/src/digest.rs b/rust/bssl-crypto/src/digest.rs
index 0ef0e8b..75c9867 100644
--- a/rust/bssl-crypto/src/digest.rs
+++ b/rust/bssl-crypto/src/digest.rs
@@ -28,6 +28,19 @@
 //! let digest2: [u8; 32] = ctx.digest();
 //!
 //! assert_eq!(digest, digest2);
+//!
+//! // Hashing with dynamic dispatch.
+//! #[cfg(feature = "std")]
+//! {
+//!     fn update_hash(ctx: &mut dyn std::io::Write) {
+//!         ctx.write(b"hel");
+//!         ctx.write(b"lo");
+//!     }
+//!
+//!     let mut ctx = digest::Sha256::new();
+//!     update_hash(&mut ctx);
+//!     assert_eq!(ctx.digest(), digest);
+//! }
 //! ```
 
 use crate::{sealed, FfiSlice, ForeignTypeRef};