add comment to guess_txintype_from_address

lib/transaction.py

@@ -844,6 +844,15 @@ class Transaction:
 
     @classmethod
     def guess_txintype_from_address(cls, addr):
+        # It's not possible to tell the script type in general
+        # just from an address.
+        # - "1" addresses are of course p2pkh
+        # - "3" addresses are p2sh but we don't know the redeem script..
+        # - "bc1" addresses (if they are 42-long) are p2wpkh
+        # - "bc1" addresses that are 62-long are p2wsh but we don't know the script..
+        # If we don't know the script, we _guess_ it is pubkeyhash.
+        # As this method is used e.g. for tx size estimation,
+        # the estimation will not be precise.
         witver, witprog = segwit_addr.decode(constants.net.SEGWIT_HRP, addr)
         if witprog is not None:
             return 'p2wpkh'