Expand comment block to explain why we could be using a nested nvlist

Requires-builders: style

module/zfs/dsl_prop.c

@@ -896,8 +896,11 @@ dsl_props_set_sync_impl(dsl_dataset_t *ds, zprop_source_t source,
 
 		if (nvpair_type(pair) == DATA_TYPE_NVLIST) {
 			/*
-			 * dsl_prop_get_all_impl() returns properties in this
-			 * format.
+			 * This usually happens when we reuse the nvlist_t data
+			 * returned by the counterpart dsl_prop_get_all_impl().
+			 * For instance we do this to restore the original
+			 * received properties when an error occurs in the
+			 * zfs_ioc_recv() codepath.
 			 */
 			nvlist_t *attrs = fnvpair_value_nvlist(pair);
 			pair = fnvlist_lookup_nvpair(attrs, ZPROP_VALUE);