docs(VariantServiceProvider): ResolveVariantService

- added comments

Closes #564

Author: Минин Степан Александрович

src/Microsoft.FeatureManagement/VariantServiceProvider.cs

@@ -77,13 +77,17 @@ private async Task<TService> ResolveByStatusAsync(CancellationToken cancellation
 
         private TService ResolveVariantService(object variantKey)
         {
+            //
+            // Prefer keyed resolution when supported. This enables lazy instantiation of variant implementations.
             if (TryGetKeyedVariantService(variantKey, out var keyedVariantService))
             {
                 return keyedVariantService;
             }
 
             if (variantKey is string variantName)
             {
+                //
+                // Fall back to scanning non-keyed registrations and matching by VariantServiceAliasAttribute or type name.
                 return GetVariantServiceFallback(variantName);
             }