Mention parquet compatibility under TimeUnit.

`TimeUnit::Second` will not store a logical type when converted to parquet format.

src/datatypes/mod.rs

@@ -191,6 +191,11 @@ impl UnionMode {
 }
 
 /// The time units defined in Arrow.
+///
+/// # Parquet Compatibility
+/// As of [parquet-format v2.9](https://github.com/apache/parquet-format/blob/master/LogicalTypes.md)
+/// `TimeUnit::Second`' is not a defined parquet logical type. The use of `TimeUnit::Second` will
+/// result in no logical type being stored within a parquet file.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde_types", derive(Serialize, Deserialize))]
 pub enum TimeUnit {