Notebook(Alerts): MailGun Alert - Placeholder for domain value would be helpful

## About the bug

**Steps to reproduce:**
List the minimal actions needed to reproduce the behavior.

1. In MailGun setup an account and then an Email API integration using the Web API and Curl. Copy and save the API Key and domain for later use in Influxdb UI.
2. Go to Influxdbv2 UI (note that during this test the feature flag 'notebooksNewEndpoints' was set)
3. Add test data to a bucket
4. Create an Alert Notebook
5. Use the test data to define the query
6. In the alert section choose MailGun
7. Add the key and domain from the first step to the configuration

**Expected behavior:**

Expect that placeholder values should be used for inputs, where there may be ambiguity or lack of clarity as to the form the value should have.

**Actual behavior:**

Testing MailGun for the second time, I'm reusing a Domain value generated a couple of weeks ago.

I pasted into the input the value
```
sandbox22c4e22d38e64d8a8559ed2319031306
```
And then the Test Alert/Endpoint request failed.

The Domain value needed to be:

```
sandbox22c4e22d38e64d8a8559ed2319031306.mailgun.org
```
A placeholder that included the the parts "mailgun.org", e.g. `.mailgun.org` would have been a helpful reminder as to what the Domain value should look like.

**Visual Proof:**


## About your environment

**Environment info:**

testing in K8SIDPE Remocal

latest commit in project UI
```
ommit 7f6a7997396b4cbae41cb300cafaac4306e73b56 (HEAD -> master, upstream/master, origin/master, origin/HEAD)
Author: wiedld
Date: Fri Feb 18 16:40:49 2022 -0800
```

latest commit in K8SIDPE
```
commit eed93bdff4fb9e3247a148837cda4cd6204c69ca (HEAD -> master, origin/master, origin/alchemy/automatic, origin/HEAD)
Author: Hercules Mango Churchill
Date: Wed Feb 23 08:21:44 2022 +0000
```