document the difference between float32 and Float32, etc.

[johnnychen94]

We need to advertise the usage of lower-cased form as it's simpler and more generic:

float32.(img) # promoting the storage type while preserving its colorant

Gray{Float32}.(img) # this could accidentally convert an RGB image to Gray image

Ref: JuliaImages/ImageQualityIndexes.jl#20