add comments to describe the usage of functions to judge the port type

Signed-off-by: Kebo Liu <kebol@nvidia.com>

sfputil/main.py

@@ -291,7 +291,11 @@ def is_sfp_present(port_name):
     return bool(presence)
 
 
-# Determine whether it is a RJ45 port
+# Below defined two flavors of functions to determin whether a port is a RJ45 port.
+# They serve different types of SFP utilities. One type of SFP utility consume the
+# info stored in the STATE_DB, these utilities shall call 'is_rj45_port_from_db'
+# to judge the port type. Another type of utilities will call the platform API
+# directly to access SFP, for them shall use 'is_rj45_port_from_api'.
 def is_rj45_port_from_db(port_name, db):
     intf_type = db.get(db.STATE_DB, 'TRANSCEIVER_INFO|{}'.format(port_name), 'type')
     return intf_type == RJ45_PORT_TYPE