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.
I would really rather not -- my preference is very much for the `std.random` documentation to use precise statistical terminology as much as possible.  Do you think it's likely to be that confusing for users?

Tho' I suppose for precision's sake I should probably write, `Random variate drawn from the uniform distribution` ... :-)