chore: checkpoint before Python removal

vendor/backon/src/docs/examples/with_mut_self.md

@@ -0,0 +1,36 @@
+Retry an async function which takes `&mut self` as receiver.
+
+This is a bit more complex since we need to capture the receiver in the closure with ownership. backon supports this use case by `RetryableWithContext`.
+
+```rust
+ use anyhow::Result;
+ use backon::ExponentialBuilder;
+ use backon::RetryableWithContext;
+
+ struct Test;
+
+ impl Test {
+     async fn fetch(&mut self, url: &str) -> Result<String> {
+         Ok(reqwest::get(url).await?.text().await?)
+     }
+ }
+
+ #[tokio::main(flavor = "current_thread")]
+ async fn main() -> Result<()> {
+     let test = Test;
+
+     let (_, result) = (|mut v: Test| async {
+         let res = v.fetch("https://www.rust-lang.org").await;
+         // Return input context back.
+         (v, res)
+     })
+     .retry(ExponentialBuilder::default())
+     // Passing context in.
+     .context(test)
+     .when(|e| e.to_string() == "retryable")
+     .await;
+
+     println!("fetch succeeded: {}", result.unwrap());
+     Ok(())
+ }
+```