unhide and add docs (#2703)

Author: jcollins-g

lib/src/dartdoc_options.dart

@@ -1608,9 +1608,10 @@ Future<List<DartdocOption<Object>>> createDartdocOptions(
         hide: true),
     DartdocOptionArgOnly<bool>('showStats', false, resourceProvider,
         help: 'Show statistics useful for debugging.', hide: true),
-    // TODO(jdkoren): Unhide when we have good support for another format.
     DartdocOptionArgOnly<String>('format', 'html', resourceProvider,
-        hide: true),
+        help: 'The format of documentation to generate: `md` for markdown, '
+            '`html` for html.',
+        hide: false),
     // TODO(jcollins-g): refactor so there is a single static "create" for
     // each DartdocOptionContext that traverses the inheritance tree itself.
     ...await createExperimentOptions(resourceProvider),