docs(upgrade): document upgrade options (#1847)

## Summary

- Improve the `vp upgrade` guide with concise examples for common
upgrade flows.
- Document checking for updates, installing a specific version, and
using a custom registry.

---------

Co-authored-by: MK (fengmk2) <fengmk2@gmail.com>

Author: jong-kyung

docs/guide/upgrade.md

@@ -14,8 +14,10 @@ You can upgrade both of them independently.
 ## Global `vp`
 
 ```bash
-vp upgrade              # upgrade to the latest version
-vp upgrade --check      # check for updates without installing
+vp upgrade                        # upgrade to the latest version
+vp upgrade --check                # check for updates without installing
+vp upgrade <version>              # install a specific version
+vp upgrade --registry <registry>  # use a custom npm registry
 ```
 
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