add note about MacOS malloc warnings for tsan build

docs/debugging.md

@@ -299,6 +299,22 @@ make -j 8
 make install
 ```
 
+!!! note
+
+    On MacOS, you may see messages like this when you start Python:
+
+    ```
+    python(7027,0x1f6dfc240) malloc: nano zone abandoned due to inability to reserve vm space.
+    ```
+
+    This message is being emmitted by the MacOS malloc implementation. As
+    [explained
+    here](https://stackoverflow.com/questions/64126942/malloc-nano-zone-abandoned-due-to-inability-to-preallocate-reserved-vm-space),
+    this happens for any program compiled with thread sanitizer on MacOS and can
+    be safely ignored by setting the `MallocNanoZone` environment variable to
+    0. You should only set this in session you are running thread sanitizer
+    under, as this setting will slow down other programs that allocate memory.
+
 - To use the built Python interpreter:
 
 ```bash