Update docs to mention felt build --host (flutter#37224)

Fix to web engine build docs

Author: justinmc

lib/web_ui/README.md

@@ -25,9 +25,10 @@ get help for a specific subcommand, run `felt help SUBCOMMAND`.
 The most useful subcommands are:
 
 - `felt build` - builds a local Flutter Web engine ready to be used by the
-  Flutter framework. To use the local engine build, pass
-  `--local-engine=host_debug_unopt` to the `flutter` command, or to
-  `dev/bots/test.dart` when running a web shard, such as `web_tests`.
+  Flutter framework. To use the local engine build, build with
+  `felt build --host`, then pass `--local-engine=host_debug_unopt` to the
+  `flutter` command, or to `dev/bots/test.dart` when running a web shard, such
+  as `web_tests`.
 - `felt test` - runs web engine tests. By default, this runs all tests using
   Chromium. Passing one or more paths to specific tests would run just the
   specified tests. Run `felt help test` for more options.