clarify text alternatives vs. media alternatives for video-only content

[venkatvemuri28]

Issue Summary

• Issue Update video-only 1.2.1 Understanding document #3802: Clarify distinction between text alternatives and alternatives for time-based media in video-only prerecorded content
• Add example for complex silent animations in WCAG understanding document

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Root Cause Analysis

• Feedback in issue Update video-only 1.2.1 Understanding document #3802 highlights need for explicit explanation of text alternative (identification) vs. alternative for time-based media (full equivalent)

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Fix Details

• Inserted new paragraph in Intent section after video-only discussion
  • Explains text alternative (e.g., alt text, aria-label) for identification
  • Alternative for time-based media for full information
  • Notes flexibility for short/simple vs. complex videos
• Added new definition list item in Examples section
  • dt: "A complex silent animation"
  • dd: Describes car engine animation with brief text alternative and separate transcript link

File: understanding/20/audio-only-and-video-only-prerecorded.html

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