Add note to Vec::split_at_spare_mut docs that the method is low-level

Author: WaffleLapkin

library/alloc/src/vec/mod.rs

@@ -1837,6 +1837,21 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// [`set_len`]: Vec::set_len
     ///
+    /// Note that this is a low-level API, which should be used with care for
+    /// optimization purposes. If you need to append data to a `Vec`
+    /// you can use [`push`], [`extend`], [`extend_from_slice`],
+    /// [`extend_from_within`], [`insert`], [`append`], [`resize`] or
+    /// [`resize_with`], depending on your exact needs.
+    ///
+    /// [`push`]: Vec::push
+    /// [`extend`]: Vec::extend
+    /// [`extend_from_slice`]: Vec::extend_from_slice
+    /// [`extend_from_within`]: Vec::extend_from_within
+    /// [`insert`]: Vec::insert
+    /// [`append`]: Vec::append
+    /// [`resize`]: Vec::resize
+    /// [`resize_with`]: Vec::resize_with
+    ///
     /// # Examples
     ///
     /// ```