add paragraph about multiple threads

Author: pablogsal

peps/pep-0768.rst

@@ -329,6 +329,36 @@ feature. This way, tools can offer a useful error message explaining why they
 won't work, instead of believing that they have attached and then never having
 their script run.
 
+Multi-threading Considerations
+-----------------------------
+
+Debugging code injected through this interface executes opportunistically in
+whichever thread first encounters a safe evaluation point after the request is
+made. This behavior mirrors how Python handles signals, providing a reliable
+execution model without adding overhead. For developers needing to target
+specific threads, the debug script can be installed only on the desired thread
+structure or in all of them if needed.
+
+The Global Interpreter Lock (GIL) continues to govern execution as normal when
+debug code runs. This means if a target thread is currently executing a C
+extension that holds the GIL without releasing it, the debug code will wait
+until that operation completes and the GIL becomes available. However, the
+interface introduces no additional GIL contention beyond what the debugging
+code itself requires. Importantly, the interface remains fully compatible with
+Python's free-threaded mode, where the GIL is not held, allowing debugger code
+to execute in any available thread.
+
+In situations where all threads in the target process are blocked—waiting on I/O
+operations, sleep states, or external resources—the debugging code might not
+execute immediately. In these cases, debuggers can send a pre-registered signal
+to the process, which will interrupt the sleep state and force thread scheduling,
+creating an opportunity for the debug code to run.
+
+The execution pattern closely resembles how Python handles signals internally.
+The interpreter guarantees that debug code only runs at safe points, never
+interrupting atomic operations within the interpreter itself. This approach
+ensures that debugging operations cannot corrupt the interpreter state while
+still providing timely execution in most real-world scenarios.
 
 Backwards Compatibility
 =======================