Docs: Document the `$post` global in REST API posts and revisions con… #8590

Job	Run time
Run performance tests	10m 10s
Slack Notifications / Prepare notifications	4s
Slack Notifications / Success notifications	4s
Slack Notifications / Failure notifications	0s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Fixed notifications	0s
Failed workflow tasks	0s
	10m 18s

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