docs(nextcloud): warn about external storage (#9882)

**Description**
Adds warning about external storage

⚒️ Fixes  #

**⚙️ Type of change**

- [ ] ⚙️ Feature/App addition
- [ ] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code

**🧪 How Has This Been Tested?**

**📃 Notes:**

**✔️ Checklist:**

- [ ] ⚖️ My code follows the style guidelines of this project
- [ ] 👀 I have performed a self-review of my own code
- [ ] #️⃣ I have commented my code, particularly in hard-to-understand
areas
- [ ] 📄 I have made corresponding changes to the documentation
- [ ] ⚠️ My changes generate no new warnings
- [ ] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
- [ ] ⬆️ I increased versions for any altered app according to semantic
versioning

**➕ App addition**

If this PR is an app addition please make sure you have done the
following.

- [ ] 🪞 I have opened a PR on
[truecharts/containers](https://github.com/truecharts/containers) adding
the container to TrueCharts mirror repo.
- [ ] 🖼️ I have added an icon in the Chart's root directory called
`icon.png`

---

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Co-authored-by: StevenMcElligott <89483932+StevenMcElligott@users.noreply.github.com>

charts/stable/nextcloud/docs/migration-guide.md

@@ -2,6 +2,12 @@
 
 This guide will walk you through the migration process for Nextcloud. This guide utilize HostPath or NFS for UserData.
 
+:::danger SMB EXTERNAL STORAGE
+
+If you are using SMB authenticated external storage on your Nextcloud remove it before starting this guide. The migration process will fail if it is not removed.
+
+:::
+
 :::danger TWO FACTOR AUTHENTICATION
 
 If you are using two factor authentication on your Nextcloud accounts disable it before starting this guide. While it can be done via the CLI later it is much easier to disable it before hand.