Add comment explaining test-only workaround

tests/cases/fourslash/completionListInImportClause05.ts

@@ -9,6 +9,11 @@
 // @Filename: /node_modules/@types/c__d/index.d.ts
 ////export declare let x: number;
 
+// NOTE: When performing completion, the "current directory" appears to be "/",
+// which is well above "." (i.e. the directory containing "app.ts").  This issue
+// is specific to the virtual file system, so just work around it by putting the
+// node modules folder in "/", rather than ".".
+
 const [replacementSpan] = test.ranges();
 verify.completionsAt("1", [
     { name: "@a/b", replacementSpan },