cmd/gobind: list byte slice as a supported type.

Fixes golang/go#10481.

Change-Id: I8cee771c32347c2ef641b416dbc1967dc52a0f46
Reviewed-on: https://go-review.googlesource.com/8986
Reviewed-by: Burcu Dogan <jbd@google.com>

cmd/gobind/doc.go

@@ -43,6 +43,8 @@ Supported types include:
 
 	- String and boolean types.
 
+	- Byte slice type.
+
 	- Any function type all of whose parameters and results have
 	  supported types. Functions must return either no results,
 	  one result, or two results where the type of the second is