Actualize extgen and/or it's documentation with the GMIDL spec

[nkrapivin]

For example,
https://github.com/YoYoGames/GM-ExtensionGenerator/wiki/user_gmidl_docs#-6-optional-types
Optionals require type_hint.

However, I've been told:
foo? : string is the syntax for an optional argument of type string.

Also, the type_hint is pretty clunky to use, why do built-in types can be used in foo : T expressions but any custom type (including an enum) has to use the type_hint attribute? foo : E, foo : E[] would've made more sense, and I hope it is that way in the GMRT's GMIDL spec.