Document wxThread::Delete() behaviour more precisely

Delete() doesn't (and can't, under Unix) wait for the detached threads,
although it does do it under MSW (but arguably shouldn't), so it can't
retrieve the thread exit code there.

Document that its "rc" argument is only useful with joinable threads.

Closes #18240.

interface/wx/thread.h

@@ -1042,11 +1042,19 @@ public:
     wxThreadError Create(unsigned int stackSize = 0);
 
     /**
-        Calling Delete() gracefully terminates a @b detached thread, either when
-        the thread calls TestDestroy() or when it finishes processing.
+        Calling Delete() requests termination of any thread.
+
+        Note that Delete() doesn't actually stop the thread, but simply asks it
+        to terminate and so will work only if the thread calls TestDestroy()
+        periodically. For detached threads, Delete() returns immediately,
+        without waiting for the thread to actually terminate, while for
+        joinable threads it does wait for the thread to terminate and may also
+        return its exit code in @a rc argument.
 
         @param rc
-            The thread exit code, if rc is not NULL.
+            For joinable threads, filled with the thread exit code on
+            successful return, if non-@NULL. For detached threads this
+            parameter is not used.
 
         @param waitMode
             As described in wxThreadWait documentation, wxTHREAD_WAIT_BLOCK