Detect any attempt to use non-blocking socket from worker threads

This doesn't work, as non-blocking sockets implementation requires
dispatching events generated by them, which is only possible from the
main thread event loop, and it's better to be upfront about it rather
than failing mysteriously later.

interface/wx/socket.h

@@ -289,6 +289,12 @@ public:
     /**
         Constructor.
 
+        Notice that if the object is created from a worker thread or if it is
+        created from the main thread but the event loop is not running, @a
+        flags parameter @em must include ::wxSOCKET_BLOCK as non-blocking
+        sockets require dispatching events, which can only be done in the main
+        thread.
+
         @param flags
             Socket flags (See wxSocketBase::SetFlags())
     */