Add Union encoding documentation (#9102)

# Which issue does this PR close?

- Closes #9084.

# What changes are included in this PR?

Documentation on union types encoding in
https://arrow.apache.org/rust/arrow_row/struct.RowConverter.html.

# Are these changes tested?

Yes.

# Are there any user-facing changes?

Yes. https://arrow.apache.org/rust/arrow_row/struct.RowConverter.html
will get updated.

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: EduardAkhmetshin

arrow-row/src/lib.rs

@@ -415,6 +415,41 @@ mod variable;
 ///
 ///```
 ///
+/// ## Union Encoding
+///
+/// A union value is encoded as a single type-id byte followed by the row encoding of the selected child value.
+/// The type-id byte is always present; union arrays have no top-level null marker, so nulls are represented by the child encoding.
+///
+/// For example, given a union of Int32 (type_id = 0) and Utf8 (type_id = 1):
+///
+/// ```text
+///                           ┌──┬──────────────┐
+///  3                        │00│01│80│00│00│03│
+///                           └──┴──────────────┘
+///                            │  └─ signed integer encoding (non-null)
+///                            └──── type_id
+///
+///                           ┌──┬────────────────────────────────┐
+/// "abc"                     │01│02│'a'│'b'│'c'│00│00│00│00│00│03│
+///                           └──┴────────────────────────────────┘
+///                            │  └─ string encoding (non-null)
+///                            └──── type_id
+///
+///                           ┌──┬──────────────┐
+/// null Int32                │00│00│00│00│00│00│
+///                           └──┴──────────────┘
+///                            │  └─ signed integer encoding (null)
+///                            └──── type_id
+///
+///                           ┌──┬──┐
+/// null Utf8                 │01│00│
+///                           └──┴──┘
+///                            │  └─ string encoding (null)
+///                            └──── type_id
+/// ```
+///
+/// See [`UnionArray`] for more details on union types.
+///
 /// # Ordering
 ///
 /// ## Float Ordering
@@ -431,6 +466,12 @@ mod variable;
 /// The encoding described above will order nulls first, this can be inverted by representing
 /// nulls as `0xFF_u8` instead of `0_u8`
 ///
+/// ## Union Ordering
+///
+/// Values of the same type are ordered according to the ordering of that type.
+/// Values of different types are ordered by their type id.
+/// The type_id is negated when descending order is specified.
+///
 /// ## Reverse Column Ordering
 ///
 /// The order of a given column can be reversed by negating the encoded bytes of non-null values