docs: explain the diagnostic events that are emitted in sandbox

docs/getting-started/hello-world.mdx

@@ -352,23 +352,72 @@ soroban contract invoke \
     --to friend
 ```
 
-The following output should appear.
+The contract invocation should result in the following output.
 
 ```json
 ["Hello", "friend"]
 ```
 
 :::info The `--` double-dash is required!
+
 This is a general [CLI pattern](https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean) used by other commands like [cargo run](https://doc.rust-lang.org/cargo/commands/cargo-run.html). Everything after the `--`, sometimes called [slop](https://github.com/clap-rs/clap/issues/971), is passed to a child process. In this case, `soroban contract invoke` builds an _implicit CLI_ on-the-fly for the `hello` method in your contract. It can do this because Soroban SDK embeds your contract's schema / interface types right in the `.wasm` file that gets deployed on-chain. You can also try:
 
-    soroban contract invoke ... -- --help
+```bash
+soroban contract invoke ... -- --help
+```
 
 and
 
-    soroban contract invoke ... -- hello --help
+```bash
+soroban contract invoke ... -- hello --help
+```
 
 :::
 
+### Diagnostic Events
+
+You will also notice here (and with any other contract run in the sandbox environment) that some diagnostic output will be displayed on the screen. This is because the `soroban-cli` automatically sets the diagnostic level to "Debug" for contracts running in the sandbox.
+
+Here, we'll see two diagnostic events were emitted by our contract invocation:
+
+1. First is a `fn_call` diagnostic event. It tells you the _kind_ of event, the contract ID that will be called, the name of the function being called, and a vector of the arguments passed to the function. A (greatly simplified) version of this event's body would look like this:
+
+```
+{
+    event: {
+        body: {
+            topics: [
+                Symbol(fn_call),
+                Bytes(0000000000000000000000000000000000000000000000000000000000000001),
+                Symbol(hello),
+            ],
+            data: Symbol(friend),
+        }
+    }
+}
+```
+
+2. Next is a `fn_return` diagnostic event. These events are emitted when a contract call completes and contains the _kind_ of event, the name of the function that is returning, and the value returned by the function. A (greatly simplified) version of this event's body would look like this:
+
+```
+{
+    event: {
+        body: {
+            topics: [
+                Symbol(fn_return),
+                Symbol(hello),
+            ],
+            data: [
+                Symbol(Hello),
+                Symbol(friend),
+            ]
+        }
+    }
+}
+```
+
+You can learn more about diagnostic events in our [Events article](../fundamentals-and-concepts/events#what-are-diagnosticevents).
+
 ## Optimizing Builds
 
 Use `soroban contract optimize` to further minimize the size of the `.wasm`. First, re-install soroban-cli with the `opt` feature: