extend ExampleLinter comment with the LintStdin

Author: alexandear

example_test.go

@@ -34,6 +34,7 @@ func ExampleLinter() {
 	// - LintFiles: Check the given multiple files
 	// - LintDir: Check all workflow files in the given single directory recursively
 	// - LintRepository: Check all workflow files under .github/workflows in the given repository
+	// - LintStdin: Check the given workflow content read from STDIN
 	// - Lint: Check the given workflow content assuming the given file path
 	errs, err := l.LintFile(f, nil)
 	if err != nil {