knowledge/document_reindex/views/knowledge_config_settings.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="view_knowledge_configuration" model="ir.ui.view">
            <field name="model">knowledge.config.settings</field>
            <field name="inherit_id" ref="knowledge.view_knowledge_configuration" />
            <field name="arch" type="xml">
                <xpath expr="//group/div" position="inside">
                    <div attrs="{'invisible': [('document_reindex_unindexed', '=', True)]}">
                        <field name="document_reindex_all" class="oe_inline" />
                        <label for="document_reindex_all" />
                    </div>
                    <div attrs="{'invisible': [('document_reindex_all', '=', True)]}">
                        <field name="document_reindex_unindexed" class="oe_inline" />
                        <label for="document_reindex_unindexed" />
                    </div>
                </xpath>
            </field>
        </record>
    </data>
</openerp>