Update "Custom Mapping Types" documentation for a fully working example

Author: fnash

docs/en/reference/custom-mapping-types.rst

@@ -20,8 +20,6 @@ a ``DateTimeImmutable`` when the data is read from the database.
 
     namespace My\Project\Types;
 
-    use DateTimeImmutable;
-    use DateTimeZone;
     use Doctrine\ODM\MongoDB\Types\ClosureToPHP;
     use Doctrine\ODM\MongoDB\Types\Type;
     use MongoDB\BSON\UTCDateTime;
@@ -31,20 +29,25 @@ a ``DateTimeImmutable`` when the data is read from the database.
         // This trait provides default closureToPHP used during data hydration
         use ClosureToPHP;
 
-        public function convertToPHPValue($value): DateTimeImmutable
+        public function convertToPHPValue($value): \DateTimeImmutable
         {
-            $timeZone = new DateTimeZone($value['tz']);
+            $timeZone = new \DateTimeZone($value['tz']);
             $dateTime = $value['utc']
                 ->toDateTime()
                 ->setTimeZone($timeZone);
 
-            return DateTimeImmutable::createFromMutable($dateTime);
+            return \DateTimeImmutable::createFromMutable($dateTime);
         }
 
         public function convertToDatabaseValue($value): array
         {
-            if (! isset($value['utc'], $value['tz'])) {
-                throw new RuntimeException('Database value cannot be converted to date with timezone. Expected array with "utc" and "tz" keys.');
+            if (!$value instanceof \DateTimeImmutable) {
+                throw new \RuntimeException(
+                    sprintf(
+                        'Expected instance of \DateTimeImmutable, got %s',
+                        gettype($value)
+                    )
+                );
             }
 
             return [