rust: sync: document `PhantomData` in `Arc`

Add a comment explaining the relevant semantics of `PhantomData`. This
should help future readers who may, as I did, assume that this field is
redundant at first glance.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241107-simplify-arc-v2-1-7256e638aac1@gmail.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>