Document completionlist XMLDoc tag

### Describe the issue or suggestion

As far as I can tell, there is no readily available documentation for the `` XMLDoc tag. I doubt this is heavily used regardless, but its existence and function doesn't seem to be documented anywhere.

---
#### Document Details

⚠ *Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.*

* ID: 3bcb925f-ef72-9a64-3762-c0b4a96f9dc4
* Version Independent ID: d135d0a6-53d2-1567-0b1b-7aae5e07739f
* Platform ID: ce5b8237-dd26-ccaa-9d21-575b220ae10e
* Content: [Recommended XML documentation tags - C# reference](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/recommended-tags)
* Content Source: [docs/csharp/language-reference/xmldoc/recommended-tags.md](https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/xmldoc/recommended-tags.md)
* Service: **dotnet-csharp**
* Sub-service: **lang-reference**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**