Add fb-only note to example using sleep()

[qwl5004]

Please complete the information below:

Where is the problem?

The \sleep code snippet

What is the problem?

I tried to follow the \sleep code but it's not recognized in VS Code. Should this be PHP\sleep() instead?

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Please don't change anything below this point.

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• Build ID: HHVM=HHVM-4.68.0:HSL=v4.41.0:2020-08-17T20:16:46+0000:7af74eee111304c3c25319df9d95501d92a64697
• Page requested: /hack/asynchronous-operations/some-basics
• Page requested at: Mon, 24 Aug 2020 20:09:53 +0000
• Controller: GuidePageController

[fredemmott]

The example is correct; an fbonly section should perhaps be added here, as there is Facebook-specific behavior that’s relevant