Rollup merge of rust-lang#131809 - collinoc:fix-retain-mut-docs, r=jhpratt

Fix predicate signatures in retain_mut docs

This is my first PR here so let me know if I'm doing anything wrong.

The docs for `retain_mut` in `LinkedList` and `VecDeque` say the predicate takes `&e`, but it should be `&mut e` to match the actual signature. `Vec` [has it documented](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.retain_mut) correctly already.

Author: jhpratt

library/alloc/src/collections/linked_list.rs

@@ -1082,7 +1082,7 @@ impl<T, A: Allocator> LinkedList<T, A> {
 
     /// Retains only the elements specified by the predicate.
     ///
-    /// In other words, remove all elements `e` for which `f(&e)` returns false.
+    /// In other words, remove all elements `e` for which `f(&mut e)` returns false.
     /// This method operates in place, visiting each element exactly once in the
     /// original order, and preserves the order of the retained elements.
     ///

library/alloc/src/collections/vec_deque/mod.rs

@@ -2122,7 +2122,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
 
     /// Retains only the elements specified by the predicate.
     ///
-    /// In other words, remove all elements `e` for which `f(&e)` returns false.
+    /// In other words, remove all elements `e` for which `f(&mut e)` returns false.
     /// This method operates in place, visiting each element exactly once in the
     /// original order, and preserves the order of the retained elements.
     ///