Silence clippy lint and add note

Author: molpopgen

src/trees.rs

@@ -21,6 +21,11 @@ use std::ptr::NonNull;
 /// Wrapper around `tsk_tree_t`.
 pub struct Tree<'treeseq> {
     pub(crate) inner: mbox::MBox<ll_bindings::tsk_tree_t>,
+    // NOTE: this reference exists becaust tsk_tree_t
+    // contains a NON-OWNING pointer to tsk_treeseq_t.
+    // Thus, we could theoretically cause UB without
+    // tying the rust-side object liftimes together.
+    #[allow(dead_code)]
     treeseq: &'treeseq TreeSequence,
     api: TreeInterface,
     current_tree: i32,