Fix missing unittests in docs

[ntrel]

Found using dlang/dmd#14527.

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[ntrel]

Updated. There are some more to fix but I've stalled on this right now so marked as ready.

[ntrel]

I think private undocumented symbols should be ignored when matching a documented unittest to a symbol (https://issues.dlang.org/show_bug.cgi?id=17678). So I've removed four changes that should be unnecessary if that is implemented.