Add a section about usage of data-val-msg in jquery-validation-unobtrusive

### Description

This input is expected to already be sanitized and encoded if necessary as it's going to be inserted into the DOM as HTML without further encoding.

Users relying on jquery-validation-unobtrusive should not define an error message containing user input without properly encoding it before they pass it on to jquery-validation-unobtrusive as part of `data-val-*-msg` attribute on an HTML element.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/mvc/models/validation?view=aspnetcore-8.0

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/mvc/models/validation.md

### Document ID

615db388-a244-41f7-21b5-a4085d9ed479

### Article author

@tdykstra

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[Associated WorkItem - 295454](https://dev.azure.com/msft-skilling/Content/_workitems/edit/295454)