Clarify that ComponentHub.OnRenderCompleted's errorMessage is untrusted

[SteveSandersonMS]

From #11791

Currently, ComponentHub.OnRenderCompleted accepts an errorMessage from the client, and logs it.

We should clarify this by changing the string that gets logged to something like Error message reported by untrusted client code: {errorMessage} instead of just {errorMessage}.

[javiercn]

We didn't change the string message but we added it to the docs. Covered as part of https://github.com/aspnet/AspNetCore.Docs/pull/13962/files