docs: add comments.

crates/oxc_allocator/src/clone_in.rs

@@ -1,4 +1,21 @@
 use crate::{Allocator, Box, Vec};
+
+/// A trait to explicitly clone an object into an arena allocator.
+///
+/// As a convention `Cloned` associated type should always be the same as `Self`,
+/// It'd only differ in the lifetime, Here's an example:
+///
+/// ```
+/// impl<'old_alloc, 'new_alloc> CloneIn<'new_alloc> for Struct<'old_alloc> {
+///     type Cloned = Struct<'new_alloc>;
+///     fn clone_in(&self, alloc: &'new_alloc Allocator) -> Self::Cloned {
+///         Struct { a: self.a.clone_in(alloc), b: self.b.clone_in(alloc) }
+///     }
+/// }
+/// ```
+///
+/// Implementations of this trait on non-allocated items short-circuit to `Clone::clone`.
+///
 pub trait CloneIn<'new_alloc>: Sized {
     type Cloned;