Add a line to pub usage output explaining how to see global options.

sdk/lib/_internal/pub/lib/src/command.dart

Index: sdk/lib/_internal/pub/lib/src/command.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command.dart b/sdk/lib/_internal/pub/lib/src/command.dart
index ccd54d0ea3d66c24aa87c4f618087b6223f0adf0..9ebf520897b3767df0293e2abdb691aaf0412158 100644
--- a/sdk/lib/_internal/pub/lib/src/command.dart
+++ b/sdk/lib/_internal/pub/lib/src/command.dart
@@ -248,8 +248,9 @@ abstract class PubCommand {
       buffer.write(_listCommands(subcommands));
     }
 
+    buffer.writeln();
+    buffer.writeln('Use "pub help" to see global options.');
     if (docUrl != null) {
-      buffer.writeln();
       buffer.writeln("See $docUrl for detailed documentation.");
     }