Add Params/Returns ddoc to free functions in std.random

The existing docs have not been tweaked in any way, so the new additions result in some duplication of information.  This seems a better first step than a more major revision.

Documentation for struct methods has been left unchanged, and will be addressed in a later patch.

I've also left the documentation for `uniformDistribution` unchanged as IMHO this function is so broken in its design, it really ought to be deprecated and removed :-)

Note that some of the parameter documentation will run into issues caused by https://issues.dlang.org/show_bug.cgi?id=13270.
Can we use `value` instead of `variate` here? I see it's actually a [proper term](http://en.wikipedia.org/wiki/Random_variate), but few people would know about this.