Add docstring for instrument_connection for mysql

Author: beijiez

instrumentation/opentelemetry-instrumentation-mysql/src/opentelemetry/instrumentation/mysql/__init__.py

@@ -33,6 +33,12 @@
     cursor.execute("INSERT INTO test (testField) VALUES (123)")
     cursor.close()
     cnx.close()
+    
+    cnx = MySQLInstrumentor().instrument_connection(cnx)
+    cursor = cnx.cursor()
+    cursor.execute("INSERT INTO test (testField) VALUES (123)")
+    cursor.close()
+    cnx.close()
 
 API
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