added package documentation

R=r, rsc
http://go/go-review/1025023

src/pkg/go/doc/doc.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// The doc package extracts source code documentation from a Go AST.
 package doc
 
 import (