Add docs for updating release git tags manually. (#19637)

I can't find a way to pick an older commit via the "Branches" and
"Recent Commits" UI, so just handle tag updating from the CLI:

![image](https://github.com/user-attachments/assets/ca3c3ebb-0084-4b43-ae3a-cf744efac19e)

docs/website/docs/developers/general/release-management.md

@@ -63,10 +63,19 @@ request that some feature make the cut.
 
     * Set the tag to be created and select a target commit. For example, if the
         candidate release was tagged `iree-3.1.0rc20241119` at commit `3ed07da`,
-        set the new release tag `iree-3.1.0` and use the same commit.
+        set the new release tag `v3.1.0` and use the same commit.
 
         ![rename_tag](./release-tag.png)
 
+        If the commit does not appear in the list, create and push a tag
+        manually:
+
+        ```bash
+        git checkout iree-3.1.0rc20250107
+        git tag -a v3.1.0 -m "Version 3.1.0 release."
+        git push upstream v3.1.0
+        ```
+
     * Set the title to `Release vX.Y.Z`.
 
     * Paste the release notes from the release tracking issue.