runtime: fix comments about inverse transform sampling in fastexprand

[EfveZombie]

The original comment incorrectly used the rate parameter (lambda=1/mean)
in place of the scale parameter (mean) for the exponential distribution.
This change corrects the probability density function and CDF derivation
to properly reflect the scale parameterization (mean) used in the
implementation.

The updated comment shows the correct PDF as exp(-x/mean)/mean and
adjusts subsequent derivations to maintain consistency with the scale
parameter convention. This matches the actual implementation where
x = -log(q) * mean is calculated.

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