Document the "multiple" attribute for select boxes

See #29

Author: lovasoa

examples/official-site/sqlpage/migrations/01_documentation.sql

@@ -228,7 +228,8 @@ INSERT INTO parameter(component, name, description, type, top_level, optional) S
     ('required', 'Set this to true to prevent the form contents from being sent if this field is left empty by the user.', 'BOOL', FALSE, TRUE),
     ('min', 'The minimum value to accept for an input of type number', 'NUMBER', FALSE, TRUE),
     ('max', 'The minimum value to accept for an input of type number', 'NUMBER', FALSE, TRUE),
-    ('checked', 'Used only for checkboxes. Indicates whether the checkbox should appear as already checked.', 'BOOL', FALSE, TRUE),
+    ('checked', 'Used only for checkboxes and radio buttons. Indicates whether the checkbox should appear as already checked.', 'BOOL', FALSE, TRUE),
+    ('multiple', 'Used only for select elements. Indicates that multiple elements can be selected simultaneously.', 'BOOL', FALSE, TRUE),
     ('step', 'The increment of values in an input of type number. Set to 1 to allow only integers.', 'NUMBER', FALSE, TRUE),
     ('description', 'A helper text to display near the input field.', 'TEXT', FALSE, TRUE),
     ('pattern', 'A regular expression that the value must match. For instance, [0-9]{3} will only accept 3 digits.', 'TEXT', FALSE, TRUE),