docs: Update SliceConcatExt docs for assoc types

Author: Ulrik Sverdrup

src/libcollections/slice.rs

@@ -999,7 +999,7 @@ impl<T> [T] {
 #[unstable(feature = "collections", reason = "U should be an associated type")]
 /// An extension trait for concatenating slices
 pub trait SliceConcatExt<T: ?Sized, U> {
-    /// Flattens a slice of `T` into a single value `U`.
+    /// Flattens a slice of `T` into a single value `Self::Output`.
     ///
     /// # Examples
     ///
@@ -1009,7 +1009,8 @@ pub trait SliceConcatExt<T: ?Sized, U> {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn concat(&self) -> U;
 
-    /// Flattens a slice of `T` into a single value `U`, placing a given separator between each.
+    /// Flattens a slice of `T` into a single value `Self::Output`, placing a given separator
+    /// between each.
     ///
     /// # Examples
     ///