LuaSurface::create_entity missing variant parameter for loader types

[_CodeGreen]

LuaSurface::create_entity allows the use of the `type` field for use with underground belts, and that is listed.
However, you can set "input" and "output" on loaders as well, and this isn't listed on the method doc.
I assume this is not intentional, and I'm glad I guessed that it worked and found out, but others may not be as lucky.