Improve Random docs

[gjp]

I'll have a go at clarifying this one.

[gjp]

Kernel.rand behaves differently from Random.rand in counter-intuitive ways. I'm not sure if I'll be doing people a favor by documenting the corner cases:

ruby-1.9.3-rc1 :024 > rand(1.9)
=> 0 # always!

ruby-1.9.3-rc1 :031 > rand(-0.1)
=> 0.9186990608553834