random.3: Clarify confusing summary

random.3 is only "better" in contrast to rand.3. Both are non-cryptographic
pseudo-random number generators. The opening blurbs of each's DESCRIPTION
section does emphasize this, and correctly directs unfamiliar developers to
arc4random(3). However, the summary (".Nd" or Name description) of random.3
conflicted in tone and message with that warning.

Resolve the conflict by clarifying in the Nd section that random(3) is
non-cryptographic and pseudo-random. Elide the "better" qualifier which
implied a comparison but did not provide a specific object to contrast.

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