Keep the concurrency hint inside the width it has

"(4 on this machine)" sits in whatever the two spinners leave of a fixed 400px
pane. That fit on one machine and ellipsed to "(4 on this machi..." on another,
which left the hint existing only in the tooltip on exactly the machines it was
written for.

Now "(4 at a time)": six characters shorter, and short enough to fit that column
with room over rather than by a hair. Why the number is smaller than the setting
is what the tooltip is for; the number itself is what has to be legible. Classic
reads the same string into its tooltip, so both UIs move together.

Layout geometry is untouched on purpose - the spinner widths and the row structure
are the part that was reported as clean.

Source/LibationUiBase/ProcessQueue/ProcessQueueViewModel.cs

@@ -124,9 +124,16 @@ public class ProcessQueueViewModel : ReactiveObject
 	/// null when it can. Closes the gap left by bounding the control at the hard limit: the setting
 	/// keeps saying what was asked for, and this says what will happen.
 	/// </summary>
+	/// <remarks>
+	/// Kept short on purpose. It lives in whatever width is left beside the spinners on a 400px
+	/// pane, and the longer wording it replaced ("on this machine") fit on some machines and
+	/// ellipsed to "(4 on this machi..." on others - which left it existing only in the tooltip on
+	/// exactly the machines it was written for. The reason it is smaller than the setting is what
+	/// the tooltip is for; the number is what has to be legible.
+	/// </remarks>
 	public string? ConcurrencyHint
 		=> EffectiveConcurrentDownloads < MaxConcurrentDownloads
-			? $"({EffectiveConcurrentDownloads} on this machine)"
+			? $"({EffectiveConcurrentDownloads} at a time)"
 			: null;
 	public string? RunningTime { get => field; set => RaiseAndSetIfChanged(ref field, value); }
 	public bool ProgressBarVisible { get => field; set => RaiseAndSetIfChanged(ref field, value); }