Cite sources for issue which caused future to be !Send

Author: rylev

src/vision/status_quo/alan_thinks_he_needs_async_locks.md

@@ -111,6 +111,7 @@ In the end, Barbara decides to write a blog post about how blocking in async cod
 ### **What are the sources for this story?**
     * Chats with [Alice](https://github.com/Darksonn) and [Lucio](https://github.com/LucioFranco).
     * Alice's [blog post](https://ryhl.io/blog/async-what-is-blocking/) on the subject has some good insights.
+    * The issue of conservative analysis of whether values are used across await points causing futures to be `!Send` is [known](https://rust-lang.github.io/async-book/07_workarounds/03_send_approximation.html), but it takes some digging to find out about this issue. A tracking issue for this can be [found here](https://github.com/rust-lang/rust/issues/57478).
 ### **Why did you choose [Alan](../characters/alan.md) to tell this story?**
     * While Barbara might be tripped up on some of the subtlties, an experienced Rust developer can usually tell how to avoid some of the issues of using locks in async code. Alan on the other hand, might be surprised when his code does not compile as the issue the `Send` error is protecting against (i.e., a mutex guard being moved to another thread) is not protected against in other languages.
 ### **How would this story have played out differently for the other characters?**