Clarify open drain (#3181)

* Migrationguide: Clarify how to replace `OpenDrainOutput`

* Add hint to `DriveMode::OpenDrain`

Author: bjoernQ

esp-hal/MIGRATING-0.23.md

@@ -255,16 +255,17 @@ GPIO drivers now take configuration structs.
 
 The OutputOpenDrain driver has been removed. You can use `Output` instead with
 `DriveMode::OpenDrain`. The input-related methods of `OutputOpenDrain` (`level`,
-`is_high`, `is_low`) are available through the (unstable) `Flex` driver.
+`is_high`, `is_low`) are available through the (unstable) `Flex` driver when enabling input.
 
 ```diff
-- OutputOpenDrain::new(peripherals.GPIO0, Level::Low);
-+ Output::new(
-     peripherals.GPIO0,
-     Level::Low,
-     OutputConfig::default()
-         .with_drive_mode(DriveMode::OpenDrain),
- );
+- let mut gpio = OutputOpenDrain::new(peripherals.GPIO0, Level::Low);
++ let mut gpio = Output::new(
++     peripherals.GPIO0,
++     Level::Low,
++     OutputConfig::default()
++         .with_drive_mode(DriveMode::OpenDrain),
++ ).into_flex();
++ gpio.enable_input(true);
 ```
 
 ## AES DMA driver changes

esp-hal/src/gpio/mod.rs

@@ -1101,6 +1101,10 @@ pub enum DriveMode {
     /// logical [`Level`], but leaves the high level floating, which is then
     /// determined by external hardware, or internal pull-up/pull-down
     /// resistors.
+    #[cfg_attr(
+        feature = "unstable",
+        doc = "\n\nEnable the input related functionality by using [Output::into_flex] and enabling input via [Flex::enable_input]"
+    )]
     OpenDrain,
 }