Update closing_signed fee requirement

With anchor outputs, we can keep the commit tx feerate lower than the real
on-chain feerate. That means that when closing the channel, the resulting
fee will not necessarily be lower than the current commit tx fee, this
requirement doesn't make sense to be strict.

Author: t-bast

02-peer-protocol.md

@@ -555,12 +555,13 @@ The funding node:
     - SHOULD send a `closing_signed` message.
 
 The sending node:
-  - MUST set `fee_satoshis` less than or equal to the
- base fee of the final commitment transaction, as calculated in [BOLT #3](03-transactions.md#fee-calculation).
-  - SHOULD set the initial `fee_satoshis` according to its
- estimate of cost of inclusion in a block.
-  - MUST set `signature` to the Bitcoin signature of the close
- transaction, as specified in [BOLT #3](03-transactions.md#closing-transaction).
+  - If the channel does not use `option_anchor_outputs`:
+    - MUST set `fee_satoshis` less than or equal to the base fee of the final
+    commitment transaction, as calculated in [BOLT #3](03-transactions.md#fee-calculation).
+  - SHOULD set the initial `fee_satoshis` according to its estimate of cost of
+  inclusion in a block.
+  - MUST set `signature` to the Bitcoin signature of the close transaction,
+  as specified in [BOLT #3](03-transactions.md#closing-transaction).
 
 The receiving node:
   - if the `signature` is not valid for either variant of closing transaction
@@ -569,9 +570,9 @@ The receiving node:
   - if `fee_satoshis` is equal to its previously sent `fee_satoshis`:
     - SHOULD sign and broadcast the final closing transaction.
     - MAY close the connection.
-  - otherwise, if `fee_satoshis` is greater than
-the base fee of the final commitment transaction as calculated in
-[BOLT #3](03-transactions.md#fee-calculation):
+  - otherwise, if `fee_satoshis` is greater than the base fee of the final
+  commitment transaction as calculated in [BOLT #3](03-transactions.md#fee-calculation)
+  and the channel does not use `option_anchor_outputs`:
     - MUST fail the connection.
   - if `fee_satoshis` is not strictly
 between its last-sent `fee_satoshis` and its previously-received