Update _send_value docstring to explain pickling requirement

Clarify that _send_value is a static method specifically to be
picklable as a Process target function.

Author: gpshead

Lib/test/test_multiprocessing_forkserver/test_preload.py

@@ -30,7 +30,7 @@ def tearDown(self):
 
     @staticmethod
     def _send_value(conn, value):
-        """Helper to send a value through a connection."""
+        """Send value through connection. Static method to be picklable as Process target."""
         conn.send(value)
 
     def test_preload_on_error_ignore_default(self):