docs(clusterissuer): nameserver warning (#14348)

**Description**
Adds a warning that cloudflare nameserver cant not be blocked by
firewall.

⚒️ 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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---------

Signed-off-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Co-authored-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>

charts/enterprise/clusterissuer/docs/how-to.md

@@ -8,6 +8,14 @@ This guide will walk you through setting up `clusterissuer`, certificate managem
 - [Traefik](https://truecharts.org/charts/enterprise/traefik/) is installed from enterprise train
 - [Cert-Mananger](https://truecharts.org/charts/operators/cert-manager/) and [Prometheus-Operator](https://truecharts.org/charts/operators/prometheus-operator/) are installed from the operators train
 
+:::warning DNS
+
+As part of the DNS verification process cert-manager will connect to authoritative nameservers to validate the DNS ACME entry. Any firewall or router rules blocking or modifying DNS traffic will cause this process to fail and prevent the issuance of certificates. Ensure no firewall or router rules are in place blocking or modifying DNS traffic to assigned authoritative nameservers. Below is an example of cloudflare assigned authoritative nameservers (these nameservers are unique to each user).
+
+![cloudflare-nameservers](./img/cloudflare-nameservers.png)
+
+:::
+
 ## Set Scale Nameservers
 
 It is important to configure Scale with reliable nameserver to avoid issues handling DNS-01 challenges. Under Network -> Global Configuration-> Nameservers, we recommend setting 1.1.1.1/1.0.0.1 or 8.8.8.8/8.8.4.4.