lnrpc: rejects positive inbound fees by default

Positive inbound are now rejected by default. The user can enable positive
inbound fees with the option 'accept-positive-inbound-fees'.

sample-lnd.conf

@@ -438,6 +438,13 @@
 ; If true, all HTLCs will be held until they are handled by an interceptor
 ; requireinterceptor=false
 
+; If true, lnd will also allow setting positive inbound fees. By default, lnd
+; only allows to set negative inbound fees (an inbound "discount") to remain
+; backwards compatible with senders whose implementations do not yet support
+; inbound fees. Therefore, you should ONLY set this setting if you know what you
+; are doing. [experimental]
+; accept-positive-inbound-fees=false
+
 ; If true, will apply a randomized staggering between 0s and 30s when
 ; reconnecting to persistent peers on startup. The first 10 reconnections will be
 ; attempted instantly, regardless of the flag's value