lnrpc: use amount based default routing fee

lnrpc/lightning.proto

@@ -753,7 +753,8 @@ message SendRequest {
     The maximum number of satoshis that will be paid as a fee of the payment.
     This value can be represented either as a percentage of the amount being
     sent, or as a fixed amount of the maximum fee the user is willing the pay to
-    send the payment.
+    send the payment. If not specified, lnd will use a default value of 100%
+    fees for small amounts (<=50 sat) or 5% fees for larger amounts.
     */
     FeeLimit fee_limit = 8;
 
@@ -2574,7 +2575,8 @@ message QueryRoutesRequest {
     The maximum number of satoshis that will be paid as a fee of the payment.
     This value can be represented either as a percentage of the amount being
     sent, or as a fixed amount of the maximum fee the user is willing the pay to
-    send the payment.
+    send the payment. If not specified, lnd will use a default value of 100%
+    fees for small amounts (<=50 sat) or 5% fees for larger amounts.
     */
     FeeLimit fee_limit = 5;