Constructor/conversion for dimensionless quantities

For pretty much all types `T <: Number`, calling `T(x)` and `convert(T, x)` are equivalent. However, for dimensionless unitful quantities, this is not the case:
```julia
julia> Float64(5u"mm/m")
5.0 mm m^-1

julia> convert(Float64, 5u"mm/m")
0.005
```
I think this is confusing and should at least be documented.

I also think that it’s very unfortunate that the `Float64` constructor does not actually return a value of type `Float64`. I would rather have a dedicated function for converting the numerical value of a quantity to a different type. Of course, this can only be changed in 2.0.

Start by reproducing the Float64(...) and convert(Float64, ...) examples for the dimensionless quantity 5u"mm/m". Document the observed difference and clarify that changing constructor behavior is deferred to version 2.0; the issue names no file or test to update.

Written by the indexing model from the issue text.