app: make --show-playground visible in --help.
After Alexandre Prokoudine's insistent demand! :-) I am still not sure how wise this is, since this should be really considered a "developer-only" option. Basically these tools are really too buggy and unstable and we should not shine too much light on these. The counter-argument is that doing so will favor the bitrot. Well ok. At least let's add a big warning message at the top of the Playground page, to make it very clear (if that were not already the case) that basically this is not to be considered a secret feature, but really more a "we are looking for contributors" option.
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