Clarify layout requirement for tabs in navigation (#3522)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: Devin Logan <devinannlogan@gmail.com>

Author: devin-ai-integration[bot]

fern/products/docs/pages/navigation/tabs.mdx

@@ -7,7 +7,9 @@ Tabs let you group sections of your documentation together, while tab variants a
 
 ## Tabs
 
-Add `tabs` to group sections together. The example below shows tabs for `Help Center`, `API Reference`, and an external link to `Github`. Each tab has a `display-name` and `icon`. 
+Add `tabs` to group sections together. The example below shows tabs for `Help Center`, `API Reference`, and an external link to `Github`. Each tab has a `display-name` and `icon`.
+
+In the `navigation` section, each tab reference must include either a `layout` (for content) or `variants` (for [tab variants](#tab-variants)). Tabs with an `href` property are external links and must not include `layout` or `variants`.
 
 <CodeBlock>
 {/* <!-- vale off --> */}
@@ -97,8 +99,16 @@ theme:
   Hide the tab from navigation
 </ParamField>
 
+<ParamField path="layout" type="list">
+  Navigation structure for the tab's content. Required unless the tab uses `variants` or `href`.
+</ParamField>
+
+<ParamField path="variants" type="list">
+  List of [tab variants](#tab-variants) for displaying different content perspectives. Use instead of `layout`.
+</ParamField>
+
 <ParamField path="href" type="string">
-  External URL. When set, clicking the tab redirects to this URL. Tabs with `href` must not have children in the navigation.
+  External URL. When set, clicking the tab redirects to this URL. Tabs with `href` must not include `layout` or `variants`.
 </ParamField>
 
 <ParamField path="target" type="string">