Change `txvec!` doc description
As of Exonum v0.4, `Into<Box<Transaction>>` is automatically implemented
for transactions, so the specified requirement is obsolete.
// See the License for the specific language governing permissions and
// limitations under the License.
-/// Creates a `Vec<Box<Transaction>>` from the given transactions.
-///
-/// Each transaction must have an `Into<Box<Transaction>>` implementation.
+/// Creates a `Vec<Box<Transaction>>` from the given transactions, or other objects
+/// implementing the `Into<Box<Transaction>>` trait.
#[macro_export]
macro_rules! txvec {
($($x:expr),*) => (
