Improve API docs concerning sort behavior

Author: m-kuhn

python/core/auto_generated/qgis.sip.in

@@ -184,6 +184,8 @@ Compares two QVariant values and returns whether the first is less than the seco
 Useful for sorting lists of variants, correctly handling sorting of the various
 QVariant data types (such as strings, numeric values, dates and times)
 
+Invalid < NULL < Values
+
 .. seealso:: :py:func:`qgsVariantGreaterThan`
 %End
 

src/core/qgis.h

@@ -444,6 +444,9 @@ CORE_EXPORT qlonglong qgsPermissiveToLongLong( QString string, bool &ok );
  * Compares two QVariant values and returns whether the first is less than the second.
  * Useful for sorting lists of variants, correctly handling sorting of the various
  * QVariant data types (such as strings, numeric values, dates and times)
+ *
+ * Invalid < NULL < Values
+ *
  * \see qgsVariantGreaterThan()
  */
 CORE_EXPORT bool qgsVariantLessThan( const QVariant &lhs, const QVariant &rhs );