Dot11Type MainType() Function Does Not Return `Mgmt`

New to both Go and this library so, perhaps I've missed something. Here's a simple example of what I'm seeing:
```go
packet := gopacket.NewPacket(
data,
layers.LayerTypeRadioTap,
gopacket.DecodeOptions{Lazy: true, NoCopy: true},
)

// retrieve the dot11 layer
dot11Layer := packet.Layer(layers.LayerTypeDot11)
if dot11Layer == nil {
log.Fatalf("No 802.11 layer found in packet %d", i)
}
dot11, ok := dot11Layer.(*layers.Dot11)

fmt.Printf("MainType: %s\n", dot11.Type.MainType())
fmt.Printf("Type: %s\n", dot11.Type)

```
From this, I get the following output:
```
MainType: MgmtAssociationReq
Type: MgmtBeacon
```

According to the docs I would have expected the MainType value to be something like `Mgmt`. https://github.com/google/gopacket/blob/v1.1.19/layers/enums.go#L214

Inspecting the library code, I believe I see that `Mgmt` is actually never going to be returned? The `Dot11TypeMetadata` slice doesn't actuallly have an entry for `Dot11TypeMgmt`. This appears to be related to the fact that the following two values are the same?
```
...
Dot11TypeMgmt Dot11Type = 0x00
...
Dot11TypeMgmtAssociationReq Dot11Type = 0x00
...
```

So when the `MainType` function applies it's masking it can only mask to a single value? If all of that is true, it feels like there's a reason or maybe some history in why the `Mgmt` type isn't represented when calling `MainType`.

Does anyone have any insights on this? Have I missed something? Most importantly, if I want to determine the `MainType` value to be one of management, control, or data, should I code against the currently returned value of `MgmtAssociationReq`? Or should I perhaps look at the integer representations of these values instead?

Thanks for your insights.