More docs/examples for structs/constructors

[oxinabox]

How to work with struct:

Although there is a good documentation on Composite it would be cool to have some concrete examples on how to use it. For example, should a pullback return a Composite or a NamedTuple for a struct argument? Also maybe warn that by default the output from a constructor will be a Composite etc...

Constructor rules:

Similarly to the previous point, I saw a lot of confusion on how to properly define rrule for constructors and objects acting as functions. Having a small example explaining that for a constructor ::Type{Object} should be used, or directly ::Object would make it more clear

originally posted by @theogf in #313