docs: improve ancestors_between docs to make behaviour more clear

[xanderbailey]

Which issue does this PR close?

• Closes #.

Test here

iceberg-rust/crates/iceberg/src/util/snapshot.rs

Lines 173 to 180 in 3253d1c

	fn test_ancestors_between_nonexistent_oldest_returns_full_chain() {
	let meta = metadata();
	// oldest_snapshot_id doesn't exist in the chain, so take_while never stops
	let ids: Vec<i64> = ancestors_between(&meta, S5, Some(999))
	.map(|s| s.snapshot_id())
	.collect();
	assert_eq!(ids, vec![S5, S4, S3, S2, S1]);
	}

shows that if oldest_snapshot_id is not an ancestor of latest_snapshot_id then the whole chain is returned but this isn't clear to the caller without reading the implementation or the tests.

What changes are included in this PR?

Docs changes

Are these changes tested?

[xanderbailey]

@CTTY are you able to take a look here?