devapp/owners: add agnivade as secondary to go/doc

To ease Robert's load and help out with a first pass review.

Change-Id: I3e0aa1fe7211429de4352358073870c4c8be7f8e
Reviewed-on: https://go-review.googlesource.com/c/build/+/166057
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

devapp/owners/table.go

@@ -273,7 +273,8 @@ var entries = map[string]*Entry{
 		Primary: []Owner{gri},
 	},
 	"go/src/go/doc": {
-		Primary: []Owner{gri},
+		Primary:   []Owner{gri},
+		Secondary: []Owner{agnivade},
 	},
 	"go/src/go/format": {
 		Primary: []Owner{gri},