submarine swap: add comment to explain witness script asymmetry

electrum/submarine_swaps.py

@@ -47,6 +47,14 @@ WITNESS_TEMPLATE_SWAP = [
 ]
 
 
+# The script of the reverse swaps has one extra check in it to verify
+# that the length of the preimage is 32. This is required because in
+# the reverse swaps the preimage is generated by the user and to
+# settle the hold invoice, you need a preimage with 32 bytes . If that
+# check wasn't there the user could generate a preimage with a
+# different length which would still allow for claiming the onchain
+# coins but the invoice couldn't be settled
+
 WITNESS_TEMPLATE_REVERSE_SWAP = [
     opcodes.OP_SIZE,
     OPPushDataGeneric(None),