Removed documentation for field instrumentation.
Removed documentation for field instrumentation.

rmosolgo/graphql-ruby,xuorig/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,xuorig/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,xuorig/graphql-ruby