doc: add missing introduced_in metadata

Fixes doc build warnings:
```
Failed to add alternative version links to webstreams
Failed to add alternative version links to async_context
```

PR-URL: #39575
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: richardlau

doc/api/async_context.md

@@ -1,5 +1,7 @@
 # Asynchronous Context Tracking
 
+<!--introduced_in=v16.4.0-->
+
 > Stability: 2 - Stable
 
 <!-- source_link=lib/async_hooks.js -->

doc/api/webstreams.md

@@ -1,5 +1,7 @@
 # Web Streams API
 
+<!--introduced_in=v16.5.0-->
+
 > Stability: 1 - Experimental
 
 An implementation of the [WHATWG Streams Standard][].