📖 [amp story captions] Update docs

[processprocess]

Updates amp-story-captions docs for official launch.
Corrects comment in css file.

[ychsieh]

@processprocess going to merge it within 24 hours. Feel free to reply here if there are actually some pending issues that I am not aware of : )

[ampprojectbot]

Warning: disparity between this PR Percy build and its main build

The Percy build for this PR was approved (either manually by a member of the AMP team, or automatically if there were no visual diffs). However, during a continuous integration step we generated another Percy build using the commit on the main branch that this PR was merged into, and there appears to be a mismatch between the two.

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