config+lncfg+sample-lnd.conf: fix feeurl documentation

A new top level feeurl option was added recently to replace the neutrino.feeurl
option. The new option was never added to the sample config file and the
text was never updated to reflect that the option is required for
neutrino on mainnet. We fix this and also add a valid mainnet example
URL to the sample config file.

sample-lnd.conf

@@ -219,6 +219,11 @@
 ; The target location of the channel backup file.
 ; backupfilepath=~/.lnd/data/chain/bitcoin/simnet/channel.backup
 
+; Optional URL for external fee estimation. If no URL is specified, the method
+; for fee estimation will depend on the chosen backend and network. Must be set
+; for neutrino on mainnet.
+; feeurl=https://nodes.lightning.computer/fees/v1/btc-fee-estimates.json
+
 ; If true, then automatic network bootstrapping will not be attempted. This
 ; means that your node won't attempt to automatically seek out peers on the
 ; network.
@@ -540,7 +545,8 @@ bitcoin.node=btcd
 ; NOTE: This value is currently unused.
 ; neutrino.banthreshold=
 
-; Set a URL source for fee estimates.
+; DEPRECATED: Use top level 'feeurl' option. Optional URL for fee estimation. If
+; a URL is not specified, static fees will be used for estimation.
 ; neutrino.feeurl=
 
 ; Optional filter header in height:hash format to assert the state of neutrino's