Fix minor inconsistencies and errors in gdextension_interface.h

[dsnopek]

I've been working on a script to parse gdextension_interface.h (which is something a number of language bindings do!) and in the process I've discovered some minor errors and inconsistencies.

These include:

• Incorrect argument names in doc comments
• Forgetting information after @return
• Putting trailing docs after interface function definitions, when we have a whole doc block above it
• Using raw function pointer types in structs or as arguments, rather than typedefing a type for them

None of these are functional changes - only cosmetic. But just to be sure, I've run the godot-cpp tests using this modified header in both Godot and godot-cpp. Everything still works :-)

[Ivorforce]

Looks good to me.

[vnen]

I was going to comment about this but forgot. I noticed it recently when working with Xcode since it does give a bunch of warnings because of the inconsistencies.

[Repiteo]

Thanks!