Topic on Extension talk:BetaFeatures

Documented.

@MarkTraceur: Could you add documentation about the mysterious $wgBetaFeaturesWhitelist config variable?

It's documented in operations/mw-config which is the only place it's used, I believe.

@Jdforrester (WMF) it is an extension global & in the extension code so should probably be documented here in the extension docs.

I added some documentation to the page here.