docs/reference: Add note about multiple exceptions when heap is locked.

docs/reference/isr_rules.rst

@@ -43,6 +43,11 @@ for the purpose. Debugging is simplified if the following code is included in an
     import micropython
     micropython.alloc_emergency_exception_buf(100)
 
+The emergency exception buffer can only hold one exception stack trace. This means that if a second exception is
+thrown during the handling of an exception while the heap is locked, that second exception's stack trace will
+replace the original one - even if the second exception is cleanly handled. This can lead to confusing exception
+messages if the buffer is later printed.
+
 Simplicity
 ~~~~~~~~~~