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

Add copyright header to doc.go

stat/combin/combin.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package combin implements routines involving combinatorics (permutations,
-// combinations, etc.).
 package combin // import "gonum.org/v1/gonum/stat/combin"
 
 import "math"