add Rationale to assert() specification

[WalterBright]

This comes up now and then, it's nice to be able to point to the long discussions on it.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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[mdparker]

I'd prefer my suggested rewrite for the less casual language, but the two minor changes work, too.

[jmh530]

I would much rather a single simple explanation, with examples, and then the link to further discussion.

[aG0aep6G]

This is an alarmingly low-effort and low-quality PR. A discussion thread with hundreds of messages of different authors displaying different opinions does not make a good rationale.