Update settings.rst

Clarified that RANDOMIZE_DOWNLOAD_DELAY, while enabled by default, does not create any delay in requests unless DOWNLOAD_DELAY is an int > 0 as RANDOMIZE_DOWNLOAD_DELAY is a multiple of DOWNLOAD_DELAY and a random float between 0.5 and 1.5. Since any number multiplied by zero is zero, RANDOMIZE_DOWNLOAD_DELAY only actually works if DOWNLOAD_DELAY is > 0 .
I think "but does not yield any delay" is not needed.