lnutil: add rationale for MIN_FUNDING_SAT

related: https://github.com/spesmilo/electrum/issues/7169#issuecomment-812800652

electrum/lnutil.py

@@ -309,6 +309,12 @@ REDEEM_AFTER_DOUBLE_SPENT_DELAY = 30
 
 CHANNEL_OPENING_TIMEOUT = 24*60*60
 
+# Small capacity channels are problematic for many reasons. As the onchain fees start to become
+# significant compared to the capacity, things start to break down. e.g. the counterparty
+# force-closing the channel costs much of the funds in the channel.
+# Closing a channel uses ~200 vbytes onchain, feerates could spike to 100 sat/vbyte or even higher;
+# that in itself is already 20_000 sats. This mining fee is reserved and cannot be used for payments.
+# The value below is chosen arbitrarily to be one order of magnitude higher than that.
 MIN_FUNDING_SAT = 200_000
 
 ##### CLTV-expiry-delta-related values