docs: Describe Table order based on concepts

Avoid implementation which can go stale, like it did here.

crates/toml/src/table.rs

@@ -7,8 +7,11 @@ use crate::map::Map;
 use crate::Value;
 
 /// Type representing a TOML table, payload of the `Value::Table` variant.
-/// By default it is backed by a BTreeMap, enable the `preserve_order` feature
-/// to use a LinkedHashMap instead.
+///
+/// By default it entries are stored in
+/// [lexicographic order](https://doc.rust-lang.org/std/primitive.str.html#impl-Ord-for-str)
+/// of the keys. Enable the `preserve_order` feature to store entries in the order they appear in
+/// the source file.
 pub type Table = Map<String, Value>;
 
 impl Table {