all: use fixdocs tool to store package documentation in doc.go

Add copyright header to doc.go

Keep original comment style (e.g. line comments and block comments).

graph/encoding/graphql/graphql.go

@@ -2,6 +2,4 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package graphql implements JSON marshaling and unmarshaling of graph as
-// used by GraphQL
 package graphql // import "gonum.org/v1/gonum/graph/encoding/graphql"