float(::Type) and real(::Type) are not documented

[stevengj]

float(Int) == Float64 and real(Complex{Int}) == Int. This has worked since Julia 0.4, but doesn't seem to be documented.

[oxinabox]

Related: #18596
Not the same, but related;
since the current docs for float are already wrong/unclear on what they do cover.

[ararslan]

complex works similarly with similarly undocumented behavior:

julia> complex(Int)
Complex{Int64}

[Tetralux]

Fixed by #19166.