docs: add a note about setting the null value for the primary key in …

…Model::update() (#6848)

* docs: add a note about setting the null value for the primary key in Model::update()

* change note to important

user_guide_src/source/models/model.rst

@@ -332,6 +332,8 @@ of the columns in a ``$table``, while the array's values are the values to save
 
 .. literalinclude:: model/016.php
 
+.. important:: If the ``$primaryKey`` field is set to ``null`` then the update will affect all records in the table.
+
 Multiple records may be updated with a single call by passing an array of primary keys as the first parameter:
 
 .. literalinclude:: model/017.php