Document the sha256t_hash_newtype direction

Since the default display direction is now forward, use

  `#[hash_newtype(backward)]`

in the rustdocs on the macro. Also add an example usage to the changelog
in case someone downstream is relying on the old default behaviour of
displaying backwards (unlikely).

hashes/CHANGELOG.md

@@ -1,3 +1,20 @@
+# unreleased
+
+* Change the default display direction of for tagged hashes to forwards. [#2707](https://github.com/rust-bitcoin/rust-bitcoin/pull/2707)
+
+  Note please this usage if you need to display backward:
+
+```rust
+  sha256t_hash_newtype! {
+    /// Test detailed explanation.
+    struct NewTypeTag = hash_str("tag");
+
+    /// A test hash.
+    #[hash_newtype(backward)]
+    struct NewTypeHash(_);
+  }
+```
+
 # 0.14.0 - 2024-03-21
 
 * Bump MSRV to Rust version 1.56.1 [#2188](https://github.com/rust-bitcoin/rust-bitcoin/pull/2188)

hashes/src/sha256t.rs

@@ -91,8 +91,8 @@ fn from_engine<T: Tag>(e: sha256::HashEngine) -> Hash<T> {
 ///     pub struct FooTag = hash_str("foo");
 ///
 ///     /// A foo hash.
-///     // Direction works just like in case of hash_newtype! macro.
+///     // Direction works just like the hash_newtype! macro.
-///     #[hash_newtype(forward)]
+///     #[hash_newtype(backward)]
 ///     pub struct FooHash(_);
 /// }
 /// ```