Add help doc

Signed-off-by: Ze Gan <ganze718@gmail.com>

dockers/docker-macsec/cli/clear/plugins/clear_macsec_counter.py

@@ -11,9 +11,12 @@ def macsec():
 
 
 @macsec.command('macsec')
-@click.option('--clean-cache', type=bool, required=False, default=False)
+@click.option('--clean-cache', type=bool, required=False, default=False, help="If the option of clean cache is true, next show commands will show the raw counters which based on the service booted instead of the last clear command.")
 def macsec_clear_counters(clean_cache):
-    """ Clear MACsec counts """
+    """ 
+        Clear MACsec counts.
+        This clear command will generated a cache for next show commands which will base on this cache as the zero baseline to show the increment of counters.
+    """
 
     if clean_cache:
         os.remove(show_macsec.CACHE_FILE)