docs: update doc for new nav logic

src/content/docs/style-guide/processes-procedures/update-left-navigation-pane.mdx

@@ -94,29 +94,32 @@ When the user goes to a page, we determine which section of the site they are on
 
 ## Add a new top-level nav
 
-To add a brand-new top-level nav, you need to create the necessary `.yml` file and related directories. 
+To add a brand-new top-level nav, you need to create the necessary `.yml` file and related directories.
 
-1. Add the new directory, sub-directories, and docs to `src/content`. 
-2. Copy an existing file from the `src/nav` directory or create a new `.yml` nav file from scratch. 
+1. Add the new directory, sub-directories, and docs to `src/content`.
+2. Copy an existing file from the `src/nav` directory or create a new `.yml` nav file from scratch.
 3. Customize the new nav file with the list of docs and directories you added.
    <Callout variant="tip">
-   You should have at least one level of nesting inside your new directory. Otherwise, doc titles in the auto-generated nav pages will render as H2 headings. For example, `src/content/level-one/level-two/doc` will work, while `src/content/level-one/doc` will produce strange formatting. 
-   </Callout> 
+     You should have at least one level of nesting inside your new directory.
+     Otherwise, doc titles in the auto-generated nav pages will render as H2
+     headings. For example, `src/content/level-one/level-two/doc` will work,
+     while `src/content/level-one/doc` will produce strange formatting.
+   </Callout>{' '}
 4. Test your build locally to ensure all the menus are working as expected.
 
 If you don't want your new nav to appear on the main landing page, add `rootNav: false` to your `.yml` file. (For an example of this, see `src/nav/style-guide.yml`.)
 
-## Use nav `YAML` to generate auto index content
+## Use nav `YAML` to populate page menus
 
-On build we automatically generate an index page for each sub-directory in `src/content` that does not already have an `index.mdx` file and that has at least one `.mdx` or `.md` file in it. When creating the page we look through each nav `YAML` file for a reference to the path to know which nav data to use for the left nav and the auto index page content.
+On build we automatically generate a page for each `.mdx` file and an index page for each sub-directory in `src/content` that does not already have an `index.mdx` file and that has at least one `.mdx` or `.md` file in it. When creating the page we look through each nav `YAML` file for a reference to the path to know which nav data to use for the left nav and the auto index page content.
 
-If an index page path is in more than one nav `YAML` file, we use the nav that matches its top level path with the beginning of index page path.
+If a page path is in more than one nav `YAML` file, we use the nav that matches its top level path with the beginning of the given page's path.
 
-### Nav requirements for auto index pages
+### Nav requirements
 
-In order to populate the auto index page with nav `YAML` data, you must add the index page path somewhere in the most relevant nav `YAML` file. This helps the auto index page plugin determine which nav to attach to the index page, but also helps the `<IndexContents />` component know which section to display on the index page itself.
+In order to attach nav `YAML` data to a page, you must add the page path somewhere in the most relevant nav `YAML` file. This helps the local nav plugin determine which nav to attach to a given page. It also helps the `<IndexContents />` component know which section to display on auto index pages.
 
-Each nav item must reference a directory path in `src/content` that contains at least one `.md` or `.mdx` file (otherwise the auto index plugin does not create a page for it).
+When adding auto index page links to nav `YAML` files, be aware that auto index pages are only created for file directories in `src/content` that contain at least one `.mdx` or `.md` file.
 
 <Callout variant="tip" title="Example">
   If this auto index page is used in `full-stack-observability.yml` and `integrations.yml`, we will attach the `integrations.yml` nav to it because:
@@ -134,3 +137,23 @@ matches this auto index page path:
 ```
 
 </Callout>
+
+If a page's path does not match any of the top-level paths in the nav `YAML` files it's referenced in (like the above example), the local nav plugin defaults to the first nav `YAML` file it sees. The order is a alphabetical sort of the top-level `title` fields.
+
+For example, if we have an .mdx file with this path that's included in two nav `YAML` files:
+
+```
+<mark>/docs/do-all-the-things</mark>/how-to-monitor-everything
+```
+
+We can't determine a "true nav" so the plugin would default to attaching the `full-stack-observability.yml` data to the page, because its `title` value is first alphabetically.
+
+```yaml fileName=src/nav/integrations.yml
+<mark>title: Integrations</mark>
+path: /docs/integrations
+```
+
+```yaml fileName=src/nav/full-stack-observability.yml
+<mark>title: Full Stack Observability</mark>
+path: /docs/full-stack-observability
+```