[FIX] product: bring back description in product views

Somehow the internal description got lost in the product views in
https://github.com/odoo/odoo/commit/af9d21b379f1dd42e83dcfa164af489c9261542d.
As the field is still used for demo data and is highly needed to
describe the product without exposing in one of the other new fields, it
is brought back.

Closes #24164

addons/product/views/product_views.xml

@@ -90,6 +90,11 @@
                             </group>
                         </page>
                         <page string="Notes" name="notes">
+                            <group name="description_internal">
+                                <group string="Description for Internal">
+                                    <field name="description" nolabel="1" placeholder="This note is only for internal purposes."/>
+                                </group>
+                            </group>
                             <group name="description">
                                 <group string="Description for Customers" attrs="{'invisible': [('sale_ok','=',False)]}">
                                     <field name="description_sale" nolabel="1" placeholder="This note will show up on sales orders."/>