fix!: leverage.MaxBorrow return zero instead of failing when there is no more to borrow

proto/umee/leverage/v1/leverage.proto

@@ -237,6 +237,8 @@ message Token {
   // borrow positions under both current and historic prices. The default value of
   // zero for this field causes current price to be used in those calculations
   // for the affected Token.
+  // The time span covered by the historic median will be:
+  //     oracle.Params.median_stamp_period * oracle.Params.historic_stamp_period * historic_medians.
   uint32 historic_medians = 19 [
     (gogoproto.moretags) = "yaml:\"historic_medians\""
   ];

proto/umee/leverage/v1/tx.proto

@@ -37,6 +37,7 @@ service Msg {
   rpc Borrow(MsgBorrow) returns (MsgBorrowResponse);
 
   // MaxBorrow allows a user to borrow the maximum amount of tokens their collateral will allow.
+  // Zero is returned if there is no more tokens to borrow.
   rpc MaxBorrow(MsgMaxBorrow) returns (MsgMaxBorrowResponse);
 
   // Repay allows a user to repay previously borrowed tokens and interest.