message: [types] update comment about transaction expiration times
  After the maximum value for transaction expiration is no
  longer constrained by a factor of 1_000_000, so update the comment
  to reflect that.
  Closes:
diff: @@ -81,10 +81,8 @@ pub struct RawTransaction {
  /// Expiration timestamp for this transaction, represented
  /// as seconds from the Unix Epoch. If the current blockchain timestamp
  /// is greater than or equal to this time, then the transaction has
  - /// expired and will be discarded. The maximum valid value is
  - /// u64::max_value() / 1_000_000 (so that it can be converted to
  - /// microseconds without overflowing); this value can be used to
  - /// indicate that a transaction does not expire.
  + /// expired and will be discarded. This can be set to a large value far
  + /// in the future to indicate that a transaction does not expire.
  expiration_timestamp_secs: u64,
  /// Chain ID of the Libra network this transaction is intended for.
  