explain once and for all the use of StaticPropertyMetadata.

The value has to be specified twice. The first for the actual value, the second is for nullability checks and type auto detection.

UPGRADING.md

@@ -35,7 +35,7 @@ If you are on version `1.x`, it is suggested to migrate directly to `3.0.0` (sin
 - Removed the abstract classes `GenericSerializationVisitor` and `GenericDeserializationVisitor`.
 - Removed deprecated method `VisitorInterface::getNavigator`, use `Context::getNavigator` instead
 - Removed deprecated method `JsonSerializationVisitor::addData`, 
-  use `::visitProperty(new StaticPropertyMetadata('', 'name', 'value'), null)` instead
+  use `::visitProperty(new StaticPropertyMetadata('', 'name', 'value'), 'value')` instead
 - Removed Propel and PhpCollection support
 - Changed default date format from ISO8601 to RFC3339  
 - Event listeners/handlers class names are case sensitive now
@@ -84,7 +84,7 @@ If you are on version `1.x`, it is suggested to migrate directly to `3.0.0` (sin
 **Deprecations** (will be removed in 3.0)
 
 - `JsonSerializationVisitor::setData` will be removed, 
-  use `::visitProperty(new StaticPropertyMetadata('', 'name', 'value'), null)` instead 
+  use `::visitProperty(new StaticPropertyMetadata('', 'name', 'value'), 'value')` instead 
 - `JsonSerializationVisitor::hasData` will be removed 
 - `VisitorInterface` is internal, use `SerializationVisitorInterface` and `DeserializationVisitorInterface` instead
 - `GraphNavigator` is internal, use `GraphNavigatorInterface` instead

src/JsonSerializationVisitor.php

@@ -166,7 +166,7 @@ public function hasData(string $key): bool
     }
 
     /**
-     * @deprecated Use visitProperty(new StaticPropertyMetadata(null, 'name', 'value'), null) instead
+     * @deprecated Use `::visitProperty(new StaticPropertyMetadata(null, 'name', 'value'), 'value')` instead
      *
      * Allows you to replace existing data on the current object element.
      *