Document two limitations with subtitle sideloading

#minor-release

PiperOrigin-RevId: 504517946

docs/_includes/media3-known-issue-box.html

+**[Known issue #{{include.issue-id}}](https://github.com/androidx/media/issues/{{include.issue-id}})** - {{ include.description }}
+{:.error}

docs/media-items.md

@@ -102,7 +102,8 @@ MediaItem mediaItem =
 
 Internally, `DefaultMediaSourceFactory` will use a `MergingMediaSource` to
 combine the content media source with a `SingleSampleMediaSource` for each
-subtitle track.
+subtitle track. `DefaultMediaSourceFactory` does not support sideloading
+subtitles for multi-period DASH.
 
 ## Clipping a media stream ##
 
@@ -152,5 +153,7 @@ Internally, `DefaultMediaSourceFactory` will wrap the content media source in an
 the player also needs to have its `DefaultMediaSourceFactory`
 [configured accordingly]({{ site.baseurl }}/ad-insertion.html#declarative-ad-support).
 
+{% include media3-known-issue-box.html issue-id="185" description="Subtitles, clipping and ad insertion are only supported if you use `DefaultMediaSourceFactory`." %}
+
 [playlist API]: {{ site.baseurl }}/playlists.html
 [`MediaItem.Builder` Javadoc]: {{ site.exo_sdk }}/MediaItem.Builder.html