docs: add more detailed note in Last Updated reference (#4780)

---------

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

docs/en/reference/default-theme-last-updated.md

@@ -2,8 +2,27 @@
 
 The update time of the last content will be displayed in the lower right corner of the page. To enable it, add `lastUpdated` options to your config.
 
-::: tip
+::: info
-You need to commit the markdown file to see the updated time.
+VitePress displays the "last updated" time using the timestamp of the most recent Git commit for each file. To enable this, the Markdown file must be committed to Git.
+
+Internally, VitePress runs `git log -1 --pretty="%ai"` on each file to retrieve its timestamp. If all pages show the same update time, it's likely due to shallow cloning (common in CI environments), which limits Git history.
+
+To fix this in **GitHub Actions**, use the following in your workflow:
+
+```yaml{4}
+- name: Checkout
+  uses: actions/checkout@v4
+  with:
+    fetch-depth: 0
+```
+
+Other CI/CD platforms have similar settings.
+
+If such options aren't available, you can prepend the `docs:build` command in your `package.json` with a manual fetch:
+
+```json
+"docs:build": "git fetch --unshallow && vitepress build docs"
+```
 :::
 
 ## Site-Level Config