Add CS1591 to WarningsNotAsErrors

Add CS1591 to the WarningsNotAsErrors list in src/Projections/Directory.Build.props so missing XML comment warnings (e.g. for CoreDragDropManager) are not treated as build errors. Also update the nearby comment block to document the CS1591 entry.

Author: Sergio0694

src/Projections/Directory.Build.props

@@ -7,13 +7,14 @@
       "CS0109: The member 'IChatMessage2.Status' does not hide an accessible member. The new keyword is not required."
       "CS0114: 'IUnSealedCustomEquals.GetHashCode()' hides inherited member 'object.GetHashCode()'. To make the current member
           override that implementation, add the override keyword. Otherwise add the new keyword."
+      "CS1591: Missing XML comment for publicly visible type or member 'CoreDragDropManager'."
       "CS0219: The variable '__retval' is assigned but its value is never used."
       "CS0628: 'ToggleSwitch.OnToggled()': new protected member declared in sealed type"
       "CS0660: 'CustomEquals2' defines operator == or operator != but does not override Object.Equals(object o)"
       "CA2257: The 'ABI.Windows.Foundation.Collections.IMapChangedEventArgs<K>.Vftbl' member on the 'ABI.Windows.Foundation.Collections.IMapChangedEventArgs<K>'
           type should be marked 'static' as 'ABI.Windows.Foundation.Collections.IMapChangedEventArgs<K>' has the 'DynamicInterfaceImplementationAttribute' applied"
     -->
-    <WarningsNotAsErrors>$(WarningsNotAsErrors);CS0108;CS0109;CS0114;CS0219;CS0628;CS0660;CA2257</WarningsNotAsErrors>
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);CS0108;CS0109;CS0114;CS1591;CS0219;CS0628;CS0660;CA2257</WarningsNotAsErrors>
 
     <!-- Generated projections surface evaluation-only ('[Experimental]') Windows Runtime APIs, so 'CS8305' is expected -->
     <NoWarn>$(NoWarn);CS8305</NoWarn>