Improve documentation of random() function.

Improve documentation of random() function.

Move random() and setseed() to a separate table, to have them grouped
together. Also add a notice that random() is not cryptographically secure.

Back-patch of commit 75fdcec14543b60cc0c67483d8cc47d5c7adf1a8 into
all supported versions, per discussion of the need to document that
random() is just a wrapper around random(3).