Update video-only 1.2.1 Understanding document #3802
Description
As a result of PR #3642 clarifying use of visible text in video-only time-based media, we receive the following supported comment:
When we update the understanding doc to clarify this position, it'd be nice to make sure we also clarify with an example that it isn't required for the text alternative and the alternative for text based media to be the same thing, and that for videos that are particularly long or complex, it might aid usability to separate them.
This issue captures the need to incorporate this into the 1.2.1 Understanding document.