Doc: improve documentation about random().

We didn't explicitly say that random() uses a randomly-chosen seed
if you haven't called setseed().  Do so.

Also, remove ref/set.sgml's no-longer-accurate (and never very
relevant) statement that the seed value is multiplied by 2^31-1.

Back-patch to v12 where set.sgml's claim stopped being true.
The claim that we use a source of random bits as seed was debatable
before 4203842a1, too, so v12 seems like a good place to stop.

Per question from Carl Sopchak.

Discussion: https://postgr.es/m/f37bb937-9d99-08f0-4de7-80c91a3cfc2e@sopchak.me

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