you must also create the actions 's record named:
product.product_template_action_custom_docs
for example
declare your menu just after
Try this:
<odoo>
<data>
<!-- your initial code in your <app>_view.xml -->
<record id="product.product_template_action_custom_docs" model="ir.actions.act_window">
<field name="name">Sale Order Custom Document</field>
<field name="res_model">product.template</field>
<field name="view_mode">tree,kanban,form</field>
<field name="view_type">form</field>
<field name="context">{"search_default_filter_to_sell":1}</field>
<field name="help" type="html">
<p> here you write the help form your form</p>
</field>
</record>
<!-- after the action, you can now paste your menu declaration
your specified "action", "id","name","sequence" and "parent"-->
<menuitem action="product.product_template_action_custom_docs"
id="sale_order_custom_document" parent="base.menu_custom" sequence="20" name="Sale Order Custom Document" />
</data>
<odoo>