document this admin theme tip - data-message on RemoveUrl

[rtpHarry]

https://github.com/OrchardCMS/Orchard/pull/5827

Use data-message to override the delete confirm dialog eg:

<a href="@Url.Action("Delete", new {commentEntry.Id, returnUrl=ViewContext.RequestContext.HttpContext.Request.ToUrlString()})" itemprop="RemoveUrl UnsafeUrl" data-message="@T("Are you sure you want to delete this contact method?")">@T("Delete")</a>

Added since v1.9.2