Add spec text for rendering suppression during transition

[khushalsagar]

Add spec text to support suppressing rendering opportunities during a View Transition. See discussion at w3c/csswg-drafts#7784.

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Parsing MDN data...
Parsing...
Generating HTML variant...
Error: missing <dfn> for topic "transition suppressing rendering" explicitly from <span> element containing "transition suppressing rendering"; previous heading contents are "8.1.7.3 Processing model"
Error: Missing reference: [CSSVIEWTRANSITIONS]
Error count: 2
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Generating SNAP variant...
Error: missing <dfn> for topic "transition suppressing rendering" explicitly from <span> element containing "transition suppressing rendering"; previous heading contents are "8.1.7.3 Processing model"
Error: Missing reference: [CSSVIEWTRANSITIONS]
Error count: 2
Saving index-snap

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[khushalsagar]

I'm waiting on the FPWD for the relevant CSS spec before this can land. But wanted to get an early review on the spec change.

This is related to the CSSWG issue here. The high level context is that for cross-document view transitions, we can use the existing concept of render-blocking to delay the start of the transition until the new Document is ready. But for same-document transitions the same behaviour (pausing rendering opportunities) needs to be explicitly defined.

One aspect I was unsure about is that suppressing rendering on a Document implies visual updates from nested Documents will also not be drawn. I'm not sure if this needs to be explicitly specified or if that's implied. Happy to file an issue if this needs more discussion.

[khushalsagar]

@jakearchibald's gonna take over porting relevant parts of the VT spec to html. So I'm closing this PR.