SIGNATURES in composite type definition

[tpapp]

I wonder if it would it make sense to allow

"""
$(SIGNATURES)
"""
Base.@kwdef struct Foo
    a
    b
end

document the constructors of Foo. This would be especially useful when there is not other explicit outer or inner constructor.

[ParadaCarleton]

Seems like a good idea, but I think a different name would be better--$SIGNATURES would be confusing for function objects that have their own signatures as well.