media::video and file format

On [the forum](https://forum.opendatakit.org/t/video-not-working/16362/4), a user discussed an issue getting an .mp4 file to play for `media::video` until after they had installed a third party application (VLC media player). Would it be worth adding a note to [form-styling/#media](https://docs.opendatakit.org/form-styling/#media) about the device needing to support a given video file type for it to work (that it's something outside of the control of ODK Collect)? And was that the root of the problem the user was experiencing?

Read the forum discussion linked in the issue, then review the media section at docs.opendatakit.org/form-styling/#media. Determine whether device support for the video file type explains the reported behavior and whether a note about that limitation belongs in the documentation; done means the issue's question is answered and the proposed documentation change is either made or clearly ruled out.

Written by the indexing model from the issue text.