message: doc: clarify order of match iterator
  Fixes
diff: @@ -249,6 +249,10 @@ impl SetMatches {
  }
  /// Returns an iterator over indexes in the regex that matched.
  + ///
  + /// This will always produces matches in ascending order of index, where
  + /// the index corresponds to the index of the regex that matched with
  + /// respect to its position when initially building the set.
  pub fn iter(&self) -> SetMatchesIter {
  SetMatchesIter((&*self.matches).into_iter().enumerate())
  }
  @@ -273,6 +277,10 @@ impl<'a> IntoIterator for &'a SetMatches {
  }
  /// An owned iterator over the set of matches from a regex set.
  +///
  +/// This will always produces matches in ascending order of index, where the
  +/// index corresponds to the index of the regex that matched with respect to
  +/// its position when initially building the set.
  pub struct SetMatchesIntoIter(iter::Enumerate<vec::IntoIter<bool>>);
  impl Iterator for SetMatchesIntoIter {
  @@ -304,6 +312,10 @@ impl DoubleEndedIterator for SetMatchesIntoIter {
  /// A borrowed iterator over the set of matches from a regex set.
  ///
  /// The lifetime `'a` refers to the lifetime of a `SetMatches` value.
  +///
  +/// This will always produces matches in ascending order of index, where the
  +/// index corresponds to the index of the regex that matched with respect to
  +/// its position when initially building the set.
  #[derive(Clone)]
  pub struct SetMatchesIter<'a>(iter::Enumerate<slice::Iter<'a, bool>>);
  