cmd/doc: fix repeated header bug added in previous CL

One too many lines was deleted, and it would print a header multiple times.
Add a test.

Change-Id: I4906b454dbb66193d515ffacf43849ffdc2dede6
Reviewed-on: https://go-review.googlesource.com/c/142937
Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/doc/doc_test.go

@@ -733,6 +733,9 @@ func TestDoc(t *testing.T) {
 				failed = true
 			}
 		}
+		if bytes.Count(output, []byte("TYPES\n")) > 1 {
+			t.Fatalf("%s: repeating headers", test.name)
+		}
 		if failed {
 			t.Logf("\n%s", output)
 		}

src/cmd/doc/pkg.go

@@ -426,6 +426,7 @@ func (pkg *Package) allDoc() {
 	printHdr := func(s string) {
 		if hdr != s {
 			pkg.Printf("\n%s\n\n", s)
+			hdr = s
 		}
 	}