capnp plugin: missing comment for union tag on groups.

Fixes this:

```
  union {  # tag bits [96, 112)
    file @6 :Void;  # bits[0, 0), union tag = 0
    struct :group {, union tag = 1      <<<=== parse error here
      dataWordCount @7 :UInt16;  # bits[112, 128)
```

N.b.
The parser also complained that the file was missing a Id attribute after bailing on this parse error (in a different schema, but the same kind of error), but in fact it did have one.

I've made no attempt at addressing that issue:
```
test/test.capnp:7:24: error: Parse error.
test/test.capnp:1:1: error: File does not declare an ID.  I've generated one for you.  Add this line to your file: @0xee8d1a7adfba730e;
```