docs(player): correct the stale "native video defaults to off" comments

DR-161 made `experimentalNativeVideo` default to on, but three comments still
described the pre-flip world and one of them was load-bearing:

- `nativeVideo.ts` labelled the store "Default off" directly above a `load()`
  that returns true when nothing is stored.
- The two PiP comments explained themselves as "what makes PiP work in the
  shipping configuration", which stopped being true when Android started
  shrinking the real ExoPlayer surface. They still describe the Linux path and
  the flag-off case, so they say that instead.
- `video_audio_codecs` justified its narrow codec list with "video does not play
  through ExoPlayer", which is no longer so on Android. The narrow list is still
  right, for a different reason now recorded: the flag is a user setting and a
  download outlives it, so only the intersection holds on both sides of the
  switch. DR-171 carries the same caveat.

No behaviour change.

src/lib/components/player/VideoPlayer.svelte

@@ -120,10 +120,12 @@
    * Keep native's picture-in-picture state in step with the `<video>` element.
    *
    * PiP is driven by the Activity, and it only ever knew about the native
-   * ExoPlayer surface — a path behind `experimentalNativeVideo`, which defaults
-   * to off. So in the shipping configuration nothing satisfied its "is a video
-   * playing?" check and the PiP button did nothing at all. Reporting the element
-   * gives it a surface it can shrink into. (UR-041, DR-160)
+   * ExoPlayer surface — a path behind `experimentalNativeVideo`, which at the
+   * time defaulted to off. So in the then-shipping configuration nothing
+   * satisfied its "is a video playing?" check and the PiP button did nothing at
+   * all. Reporting the element gives it a surface it can shrink into, and still
+   * has to: the flag defaults to on now (DR-161) but a user who turns it off is
+   * back on the element. (UR-041, DR-160, DR-161)
    */
   function reportPipVideoState() {
     if (!useHtml5Element || !videoElement) {