Spread props do not work in (auto-defined) factory functions about helix

[lilactown]

This is a limitation of factory functions. Spread props only work with the $ macro, or macros which use the $ macro (e.g. the macros defined in helix.dom).

Since factory functions are, as it says on the tin, functions, they can not do anything at compile time - which is what the $ macro does, rewrite the props at compile time to be akin to (merge {:bar "yo"} props).

The spread props behavior was created to optimize calls to React's createElement function. Factory functions give up this optimization, for ease of typing 😄

Hope that makes sense. Doc improvement pull requests are most welcome!