k_smallest: Document the equivalence with self.sorted().take(k)
Consistency with `Iterator::take` is the rationale for returning
less than `k` elements when the input is too short.
@@ -2298,6 +2298,9 @@ pub trait Itertools : Iterator {
/// The sorted iterator, if directly collected to a `Vec`, is converted
/// without any extra copying or allocation cost.
///
+ /// **Note:** This is functionally-equivalent to `self.sorted().take(k)`
+ /// but much more efficient.
+ ///
/// ```
/// use itertools::Itertools;
///
