Add a stability specification to the docs for unique and unique_by
This property is an important one, so if the methods guarantee it, that
should be specified in the documentation.
@@ -987,6 +987,9 @@ pub trait Itertools : Iterator {
/// Clones of visited elements are stored in a hash set in the
/// iterator.
///
+ /// The iterator is stable, returning the non-duplicate items in the order
+ /// in which they occur in the adapted iterator.
+ ///
/// ```
/// use itertools::Itertools;
///
@@ -1009,6 +1012,9 @@ pub trait Itertools : Iterator {
/// with the keying function `f` by hash and equality.
/// The keys are stored in a hash set in the iterator.
///
+ /// The iterator is stable, returning the non-duplicate items in the order
+ /// in which they occur in the adapted iterator.
+ ///
/// ```
/// use itertools::Itertools;
///
