Clarify documentation

Author: bkoelman

src/JsonApiDotNetCore/Resources/IResourceDefinition.cs

@@ -241,9 +241,9 @@ Task OnAddToRelationshipAsync(TResource leftResource, HasManyAttribute hasManyRe
     /// </para>
     /// </summary>
     /// <param name="leftResource">
-    /// The original resource as retrieved from the underlying data store. The indication "left" specifies that <paramref name="hasManyRelationship" /> is
-    /// declared on <typeparamref name="TResource" />. Be aware that for performance reasons, not the full relationship is populated, but only the subset of
-    /// resources to be removed.
+    /// Identifier of the left resource. The indication "left" specifies that <paramref name="hasManyRelationship" /> is declared on
+    /// <typeparamref name="TResource" />. In contrast to other relationship methods, only the left ID and only the subset of right resources to be removed
+    /// are retrieved from the underlying data store.
     /// </param>
     /// <param name="hasManyRelationship">
     /// The to-many relationship being removed from.