fix incorrect mod rounding

[simonbyrne]

mod currently gives some incorrect values when it should be exact, e.g.:

julia> mod(prevfloat(1.0),1.0)
0.0

julia> rem(prevfloat(1.0),1.0)
0.9999999999999999

This should fix those problems. I've also tried to improve the documentation of the related functions.

[jiahao]

+1

Would be nice to have these specific examples as tests

[simonbyrne]

@jiahao done.