docs(Dropdown): Update Usage tab with Combo box custom value function…

…ality (#4364)

* Adding custom value content

* Anna's feedback

src/pages/components/dropdown/usage.mdx

@@ -865,6 +865,25 @@ boxes are useful when the data populating the list may come from a database.
 </Column>
 </Row>
 
+#### Entering a custom value
+
+In a combo box, users can type in a custom or unique value that is not included
+in the list of predefined options.
+
+- If the desired option isn’t available in the list, users can start typing
+  their custom value directly in the field, filtering out options as they type.
+- To save the new custom value, click outside the field with the mouse or press
+  `Tab` or `Enter` on the keyboard.
+- Once saved, the placeholder text changes to display the custom typed value.
+
+<Row>
+<Column colLg={8}>
+
+![Entering a custom value in a default combo box.](images/dropdown-usage-11d.png)
+
+</Column>
+</Row>
+
 ## Modifiers
 
 #### Inline