Add doxygen note about metadata validation

Author: nyalldawson

python/core/metadata/qgslayermetadata.sip

@@ -30,6 +30,10 @@ class QgsLayerMetadata
  the schema definition available at resources/qgis-resource-metadata.xsd
  within the QGIS source code.
 
+ Metadata can be validated through the use of QgsLayerMetadataValidator
+ subclasses. E.g. validating against the native QGIS metadata schema can be performed
+ using QgsNativeMetadataValidator.
+
 .. versionadded:: 3.0
 %End
 

src/core/metadata/qgslayermetadata.h

@@ -44,6 +44,10 @@ class QgsMapLayer;
  * the schema definition available at resources/qgis-resource-metadata.xsd
  * within the QGIS source code.
  *
+ * Metadata can be validated through the use of QgsLayerMetadataValidator
+ * subclasses. E.g. validating against the native QGIS metadata schema can be performed
+ * using QgsNativeMetadataValidator.
+ *
  * \since QGIS 3.0
  */
 class CORE_EXPORT QgsLayerMetadata