Add docs for language types Parser:Document & Visitor:AST_node

[networkimprov]

In the docs for the language section of the module https://graphql.org/graphql-js/language/

There are two undocumented types needed by callers: Document returned by parse() https://graphql.org/graphql-js/language/#parse the AST node (mentioned as value) returned by functions in the visitor argument to visit() https://graphql.org/graphql-js/language/#visit

Also these functions' argument types aren't fully documented: print() - https://graphql.org/graphql-js/language/#print visit() - https://graphql.org/graphql-js/language/#visit

I'm using this module to obtain and manipulate a query AST, so docs for AST types would greatly help :)

Btw, I'm calling this module in a browser context, for a new GraphQL client.