Improve docs for Rust convenience methods (#7931)

* Improve docs for Rust convenience methods

Resolves #7928

* Document differences in approaches

docs/rust/implementation.md

@@ -16,13 +16,42 @@ Most service client crates are generated from [TypeSpec](https://aka.ms/typespec
 
 {% include requirement/MAY id="rust-client-convenience-separate" %} implement a separate client that provides features not described in a service specification.
 
-{% include requirement/MAY id="rust-client-convenience-wrap" %} implement a client which wraps a generated client e.g., using [newtype][rust-lang-newtype], and exposes necessary methods from the underlying client as well as any convenience methods.
+{% include requirement/MAY id="rust-client-convenience-wrap" %} implement a client which wraps a generated client e.g., using [newtype][rust-lang-newtype], and exposes necessary methods from the underlying client as well as any convenience methods. You might consider this approach if you want to effectively hide most generated methods and define replacements. You are responsible for transposing documentation and following all guidelines herein.
 
-{% include requirement/MAY id="rust-client-convenience-extension" %} define [extension methods][rust-lang-extension-methods] that call existing public methods.
+{% include requirement/MAY id="rust-client-convenience-extension" %} define [extension methods][rust-lang-extension-methods] in a trait that call existing public methods, e.g.,
+
+```rust
+pub trait SecretClientExt {
+    async fn deserialize_secret<T: serde::de::DeserializeOwned>(
+        &self,
+        name: impl AsRef<str>,
+        version: Option<impl AsRef<str>>,
+    ) -> Result<Response<T>>;
+}
+
+impl SecretClientExt for SecretClient {
+    async fn deserialize_secret<T: serde::de::DeserializeOwned>(
+        &self,
+        name: impl AsRef<str>,
+        version: Option<impl AsRef<str>>,
+    ) -> Result<T> {
+        let value = self.get_secret(name, version).await?;
+        serde_json::from_str(&value).map_err(Error::from)
+    }
+}
+```
+
+* The trait **MUST** be exported from the crate root.
+* The trait **MUST** use the name of the client it extends with an "Ext" suffix e.g., "SecretClientExt".
+* The trait **MAY** extend the [service client methods trait for mocking](introduction.md#rust-client-mocking-trait-name) e.g., `pub trait SecretClientExt: SecretClientMethods {}`.
+
+You might consider this approach if the generated methods are sufficient but you want to add convenience methods.
+
+#### Convenience Client Telemetry {#rust-client-convenience-telemetry}
 
 In all options above except if merely re-exposing public APIs without alteration:
 
-{% include requirement/MUST id="rust-client-convenience-telemetry" %} must telemeter the convenience client methods just like any service client methods.
+{% include requirement/MUST id="rust-client-convenience-telemetry-telemeter" %} must telemeter the convenience client methods just like any service client methods.
 
 ### Options Builders {#rust-client-options-builders}