Add notes to module_selector test about untested things

Author: beccadax

test/NameLookup/module_selector.swift

@@ -5,6 +5,15 @@
 
 // REQUIRES: swift_feature_ModuleSelector
 
+// FIXME: This test doesn't really cover:
+//
+// * Whether X::foo finds foos in X's re-exports
+// * Whether we handle access paths correctly
+// * Interaction with ClangImporter
+// * Cross-module overlays, when those happen
+//
+// It also might not cover all combinations of name lookup paths and inputs.
+
 import ModuleSelectorTestingKit
 
 import ctypes::bits   // FIXME: ban using :: with submodules?