Fix embassy migration guide (#2393)

esp-hal-embassy/MIGRATING-0.3.md

@@ -21,8 +21,43 @@ You no longer have to set up clocks and pass them to `esp_hal_embassy::init`.
 
      let timg0 = TimerGroup::new(peripherals.TIMG0);
 -    esp_hal_embassy::init(&clocks, timg0);
-+    esp_hal_embassy::init(timg0);
++    esp_hal_embassy::init(timg0.timer0);
 
      // ...
  }
 ```
+
+You have to specify a timer instance (that may be a `TimerGroup` timer unit
+or a `SystemTimer` alarm) or an array of `AnyTimer`s when calling `init`.
+An example of how you can set multiple timers (for example when using
+multiple executors):
+
+```rust
+use esp_hal::{
+    prelude::*,
+    timer::{
+        AnyTimer,
+        systimer::SystemTimer
+    }
+};
+
+#[esp_hal_embassy::main]
+async fn main(_spawner: Spawner) -> ! {
+    let peripherals = esp_hal::init(esp_hal::Config::default());
+
+    let timg0 = TimerGroup::new(peripherals.TIMG0);
+    let timer0: AnyTimer = timg0.timer0.into();
+    let timer1: AnyTimer = timg0.timer1.into();
+
+    // You can use either a TimerGroup timer, a SystemTimer alarm,
+    // or you can mix and match them as well.
+    let systimer = SystemTimer::new(peripherals.SYSTIMER).split::<Target>();
+    let timer2: AnyTimer = systimer.alarm0;
+
+    esp_hal_embassy::init([timer0, timer1, timer2]);
+
+    // ...
+}
+```
+
+Note that you only have to convert into `AnyTimer` if you want to use multiple timers.