doc: Suggest str::repeat over iter::repeat().take().collect()

Author: tesuji

library/core/src/iter/sources/repeat.rs

@@ -8,6 +8,9 @@ use crate::num::NonZero;
 /// Infinite iterators like `repeat()` are often used with adapters like
 /// [`Iterator::take()`], in order to make them finite.
 ///
+/// > Please prefer [`str::repeat()`] over this function if you just want
+/// > to repeat a char/string `n`th times.
+///
 /// If the element type of the iterator you need does not implement `Clone`,
 /// or if you do not want to keep the repeated element in memory, you can
 /// instead use the [`repeat_with()`] function.