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

This reverts commit 963877e5f5.

graph/formats/dot/ast/ast.go

@@ -8,6 +8,8 @@
 // This file is made available under a Creative Commons CC0 1.0
 // Universal Public Domain Dedication.
 
+// Package ast declares the types used to represent abstract syntax trees of
+// Graphviz DOT graphs.
 package ast // import "gonum.org/v1/gonum/graph/formats/dot/ast"
 
 import (