exit: document interaction with C

Author: RalfJung

library/std/src/process.rs

@@ -2294,6 +2294,20 @@ impl Child {
 /// considered undesirable. Note that returning from `main` also calls `exit`, so making `exit` an
 /// unsafe operation is not an option.)
 ///
+/// ## Safe interop with C code
+///
+/// This function is safe to call as long as `exit` is only ever invoked from Rust. However, the C
+/// standard does not permit multiple threads to call `exit` concurrently. Rust mitigates this with
+/// a lock, but if C code calls `exit`, that can still cause undefined behavior. Note that returning
+/// from `main` is equivalent to calling `exit`.
+/// Therefore, it is undefeined behavior to have two concurrent threads perform the following
+/// without synchronization:
+/// - One thread calls Rust's `exit` function or returns from Rust's `main` function
+/// - Another thread calls the C function `exit` or `quick_exit`, or returns from C's `main` function
+///
+/// Individual C implementations might provide more guarantees than the standard and permit concurrent
+/// calls to `exit`; consult the documentation of your C implementation for details.
+///
 /// ## Platform-specific behavior
 ///
 /// **Unix**: On Unix-like platforms, it is unlikely that all 32 bits of `exit`