[IMP] base,doc: add on_create and edit attribute on gantt view

related task #1929079

closes odoo/odoo#33872

Signed-off-by: Jérome Maes (jem) <jem@openerp.com>

doc/reference/views.rst

@@ -1318,7 +1318,7 @@ take the following attributes:
   name of the field that describes if the task has to be excluded
   from the consolidation
   if set to true it displays a striped zone in the consolidation line
-``create``, ``edit``
+``create``, ``edit``, ``plan``
     allows *dis*\ abling the corresponding action in the view by setting the
     corresponding attribute to ``false``. If ``create`` is enabled, a "+" button
     will be displayed while hovering each time slot to create a new record in
@@ -1392,6 +1392,10 @@ take the following attributes:
     meta-information. The ``getColor`` method to convert in a color integer is
     also available directly in the template context without using ``widget``.
 
+  ``on_create``
+  If specified when clicking the add button on the view, instead of opening a generic dialog, launch a client action.
+  this should hold the xmlid of the action (eg: ``on_create="%(my_module.my_wizard)d"``
+
 .. _reference/views/diagram:
 
 Diagram