Documentation of GraphQLScalarType is outdated

[Cito]

The documentation for the GraphQLScalarType is outdated compared with the current code. For instance, parseLiteal takes a valueNode and a variables parameter instead of just one valueAST parameter (I think that was changed in 0.12), and there are some more differences.

[overhead525]

@Cito this seems pretty straightforward. Have you completed this yet? If not, I'll get it done if you'll assign me.

[Cito]

@overhead525 no, haven't done anything about this yet. @IvanGoncharov can you assign it to him?

[yaacovCR]

Changes in #3812 would also require documentation update