encoding/gob: update documentation in doc.go for wireType

It was just out of date.

Fixes #30656

Change-Id: I1fab7dd93091865a8240769eca5dd19cdbc78b81
Reviewed-on: https://go-review.googlesource.com/c/go/+/166177Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/gob/doc.go

@@ -193,10 +193,14 @@ pair (-type id, encoded-type) where encoded-type is the gob encoding of a wireTy
 description, constructed from these types:
 
 	type wireType struct {
-		ArrayT  *ArrayType
-		SliceT  *SliceType
-		StructT *StructType
-		MapT    *MapType
+		ArrayT           *ArrayType
+		SliceT           *SliceType
+		StructT          *StructType
+		MapT             *MapType
+		GobEncoderT      *gobEncoderType
+		BinaryMarshalerT *gobEncoderType
+		TextMarshalerT   *gobEncoderType
+
 	}
 	type arrayType struct {
 		CommonType
@@ -224,6 +228,9 @@ description, constructed from these types:
 		Key  typeId
 		Elem typeId
 	}
+	type gobEncoderType struct {
+		CommonType
+	}
 
 If there are nested type ids, the types for all inner type ids must be defined
 before the top-level type id is used to describe an encoded-v.