Microsoft.Insights/webtests@2022-06-15 documentation does not provide allowed value limitations for Frequency.

**Bicep version**
Bicep CLI version 0.34.44 (1f661a23c4)

**Describe the bug**
Setting the Frequency property to anything other than **300**, **600** or **900** seconds will result in the following deployment error
```
"{
"status": "Failed",
"error": {
"code": "BadRequest",
"message": "Value cannot be null.\r\nParameter name: format"
}
}"
```

**To Reproduce**
Create an availability test and set the Frequency to anything than one of the three values above.

**Additional context**
We tried to create an availability test with a frequency of 3600 seconds (one hour) and struggled with the above deployment error.
Validate and What-If always succeed.

The documentation for this value should mention the allowed values.
Currently, the docs only provide this information:
`Frequency | Interval in seconds between test runs for this WebTest. Default value is 300. | int`

It would be even better, if the values would be limited in bicep directly.

Start with the Microsoft.Insights/webtests@2022-06-15 resource definition and locate the Frequency property documentation. Confirm the service accepts only 300, 600, and 900 seconds, then update the documentation to state those allowed values; investigate whether direct Bicep validation is supported. Done means the generated documentation reflects the limitation.

Written by the indexing model from the issue text.