Composite clarification

[mzgubic]

The docs say that Composite users should disregard the T parameter as it is an implementation detail.

However both ChainRules and Zygote do not always follow this recommendation.

There is a difference between the two constructors and it is not clear when to apply which one. Perhaps a comment could be added to the docs?

julia> xt = (1, 2)
(1, 2)

julia> ct1 = Composite{Any, typeof(xt)}(xt)
Composite{Any}(1, 2)

julia> ct2 = Composite{Any}(xt)
Composite{Any}((1, 2),)

[mzgubic]

As a consequence they behave differently when splatted:

julia> println(ct1...)
12

julia> println(ct2...)
(1, 2)

[nickrobinson251]

Xref #198