std.json: Document opEquals

[dkorpel]

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[dkorpel]

If I use == here, the generator will convert it to:

writeln(JSONValue(10)); // JSONValue(10.0)

Hence I'm calling opEquals explicitly here.

[CyberShadow]

I recall such awkward situations coming up before. Maybe it's time to add some kind of // no-writeln syntax to https://github.com/dlang/dlang.org/blob/master/ddoc/source/assert_writeln_magic.d ?

[CyberShadow]

dlang/dlang.org#3801

[dkorpel]

Or it could exempt documented unittests under opEquals