feat: Note about startViewTransition skipping if page not visible. Fix auto-playing audio in live sample

[bsmth]

Description

Adds a note to View Transitions API page that the transition is skipped if the page is hidden, small improvements to what it means for a page (document) to be hidden.

In parallel, the page visibility API has an example where the audio would auto-play when switching tabs, even if the user had no interaction with the media element before. This is not a good example to propagate, so we should gate the audio.play() call behind a playingOnHide boolean. It looks like this was raised in #26745, but we actually fixed a different example?

Related issues and pull requests

Fixes #32748
Fixes #26745

View transitions spec links:

• https://drafts.csswg.org/css-view-transitions-1/#page-visibility-change-steps
• https://drafts.csswg.org/css-view-transitions/#dom-document-startviewtransition

[github-actions]

Preview URLs

• /en-US/docs/Web/API/Page_Visibility_API
• /en-US/docs/Web/API/View_Transitions_API

[bsmth]

Thank you!