book: improve documentation about UDT values

This change aims to improve the documentation
about handling user defined types in the Rust driver.

Changes:
* Mention that fields in UDT and Rust struct must be in the same order
* Add a link to official Scylla docs about UDTs
* Improve grammar, make it clearer

Signed-off-by: Jan Ciolek <[email protected]>

docs/source/data-types/udt.md

@@ -1,13 +1,36 @@
 # User defined types
-Scylla allows users to define their own data types with named fields.\
-The driver supports this by allowing to create a custom struct with derived functionality.
+Scylla allows users to define their own data types with named fields (See [the official documentation](https://opensource.docs.scylladb.com/stable/cql/types.html#user-defined-types))\
+To use user defined types in the driver, you can create a corresponding struct in Rust, and use it to read and write UDT values.
+
 
 For example let's say `my_type` was created using this query:
 ```sql
 CREATE TYPE ks.my_type (int_val int, text_val text)
 ```
 
-To use this type in the driver create a matching struct and derive `IntoUserType` and `FromUserType`:
+To use this type in the driver, create a matching struct and derive `IntoUserType` and `FromUserType`:
+
+```rust
+# extern crate scylla;
+# async fn check_only_compiles() {
+use scylla::macros::{FromUserType, IntoUserType};
+
+// Define a custom struct that matches the User Defined Type created earlier.
+// Fields must be in the same order as they are in the database.
+// Wrapping a field in Option will gracefully handle null field values.
+#[derive(Debug, IntoUserType, FromUserType)]
+struct MyType {
+    int_val: i32,
+    text_val: Option<String>,
+}
+# }
+```
+
+> ***Important***\
+> Fields in the Rust struct must be defined in the same order as they are in the database.
+> When sending and receiving values, the driver will (de)serialize fields one after another, without looking at field names.
+
+Now it can be sent and received just like any other CQL value:
 ```rust
 # extern crate scylla;
 # use scylla::Session;
@@ -17,16 +40,12 @@ use scylla::IntoTypedRows;
 use scylla::macros::{FromUserType, IntoUserType};
 use scylla::cql_to_rust::FromCqlVal;
 
-// Define custom struct that matches User Defined Type created earlier
-// wrapping field in Option will gracefully handle null field values
 #[derive(Debug, IntoUserType, FromUserType)]
 struct MyType {
     int_val: i32,
     text_val: Option<String>,
 }
 
-// Now it can be sent and received like any other value
-
 // Insert my_type into the table
 let to_insert = MyType {
     int_val: 17,