irmin-graphql: Add missing `doc` params

[zshipko]

In order to use a tool like https://github.com/2fd/graphdoc to document the GraphQL API we will need to add a bit more context.

To do this we will need to pass a description to the doc parameter for the following functions:

• Schema.io_field
• Schema.field
• Schema.obj
• Schema.union
• Schema.scalar
• Schema.Arg.Scalar

Also, Default_type should be updated to accept a doc value