docs: add docs to go2proto (#2992)

```
$ go2proto
Usage: go2proto <package> <ref> ... [flags]

Generate a Protobuf schema from Go types.

It supports converting structs to messages, Go "sum types" to oneof fields,
and Go "enums" to Protobuf enums.

The generator works by extracting protobuf tags from the source Go types.
There are two locations where these tags must be specified:

 1. For fields using a tag in the form `protobuf:"<id>[,optional]"`.
 2. For sum types as comment directives in the form //protobuf:<id>.

An example showing all three supported types and the corresponding protobuf
tags:

    type UserType int

    const (
    	UserTypeUnknown UserType = iota
    	UserTypeAdmin
    	UserTypeUser
    )

    // Entity is a "sum type" consisting of User and Group.
    //
    // Every sum type element must have a comment directive in the form //protobuf:<id>.
    type Entity interface { entity() }

    //protobuf:1
    type User struct {
    	Name   string    `protobuf:"1"`
    	Type   UserType `protobuf:"2"`
    }
    func (User) entity() {}

    //protobuf:2
    type Group struct {
    	Users []string `protobuf:"1"`
    }
    func (Group) entity() {}

    type Role struct {
    	Name string `protobuf:"1"`
    	Entities []Entity `protobuf:"2"`
    }

And this is the corresponding protobuf schema:

    message Entity {
      oneof value {
        User user = 1;
        Group group = 2;
      }
    }

    enum UserType {
      USER_TYPE_UNKNOWN = 0;
      USER_TYPE_ADMIN = 1;
      USER_TYPE_USER = 2;
    }

    message User {
      string Name = 1;
      UserType Type = 2;
    }

    message Group {
      repeated string users = 1;
    }

    message Role {
      string name = 1;
      repeated Entity entities = 2;
    }

Arguments:
  <package>    Package name to use in the generated protobuf schema.
  <ref> ...    Type to generate protobuf schema from in the form PKG.TYPE. eg.
               github.com/foo/bar/waz.Waz or ./waz.Waz

Flags:
  -h, --help                   Show context-sensitive help.
  -o, --output=STRING          Output file to write generated protobuf schema
                               to.
  -I, --imports=IMPORTS,...    Additional imports to include in the generated
                               protobuf schema.
  -g, --go-package=STRING      Go package to use in the generated protobuf
                               schema.

go2proto: error: expected "<package> <ref> ..."
```

cmd/go2proto/main.go

@@ -20,19 +20,92 @@ import (
 	"github.com/TBD54566975/ftl/internal/schema/strcase"
 )
 
+const help = `Generate a Protobuf schema from Go types.
+
+It supports converting structs to messages, Go "sum types" to oneof fields, and Go "enums" to Protobuf enums.
+
+The generator works by extracting protobuf tags from the source Go types. There are two locations where these tags must
+be specified:
+
+  1. For fields using a tag in the form ` + "`protobuf:\"<id>[,optional]\"`" + `.
+  2. For sum types as comment directives in the form //protobuf:<id>.
+
+An example showing all three supported types and the corresponding protobuf tags:
+
+	type UserType int
+
+	const (
+		UserTypeUnknown UserType = iota
+		UserTypeAdmin
+		UserTypeUser
+	)
+
+	// Entity is a "sum type" consisting of User and Group.
+	//
+	// Every sum type element must have a comment directive in the form //protobuf:<id>.
+	type Entity interface { entity() }
+
+	//protobuf:1
+	type User struct {
+		Name   string    ` + "`protobuf:\"1\"`" + `
+		Type   UserType ` + "`protobuf:\"2\"`" + `
+	}
+	func (User) entity() {}
+
+	//protobuf:2
+	type Group struct {
+		Users []string ` + "`protobuf:\"1\"`" + `
+	}
+	func (Group) entity() {}
+
+	type Role struct {
+		Name string ` + "`protobuf:\"1\"`" + `
+		Entities []Entity ` + "`protobuf:\"2\"`" + `
+	}
+
+And this is the corresponding protobuf schema:
+
+	message Entity {
+	  oneof value {
+	    User user = 1;
+	    Group group = 2;
+	  }
+	}
+
+	enum UserType {
+	  USER_TYPE_UNKNOWN = 0;
+	  USER_TYPE_ADMIN = 1;
+	  USER_TYPE_USER = 2;
+	}
+
+	message User {
+	  string Name = 1;
+	  UserType Type = 2;
+	}
+
+	message Group {
+	  repeated string users = 1;
+	}
+
+	message Role {
+	  string name = 1;
+	  repeated Entity entities = 2;
+	}
+`
+
 type Config struct {
 	Output    string   `help:"Output file to write generated protobuf schema to." short:"o"`
 	Imports   []string `help:"Additional imports to include in the generated protobuf schema." short:"I"`
 	GoPackage string   `help:"Go package to use in the generated protobuf schema." short:"g"`
 
 	Package string   `arg:"" help:"Package name to use in the generated protobuf schema."`
-	Ref     []string `arg:"" help:"Type to generate protobuf schema from in the form PKG.TYPE. eg. github.com/foo/bar.Waz" required:"true" placeholder:"PKG.TYPE"`
+	Ref     []string `arg:"" help:"Type to generate protobuf schema from in the form PKG.TYPE. eg. github.com/foo/bar/waz.Waz or ./waz.Waz" required:"true" placeholder:"PKG.TYPE"`
 }
 
 func main() {
 	fset := token.NewFileSet()
 	cli := Config{}
-	kctx := kong.Parse(&cli)
+	kctx := kong.Parse(&cli, kong.Description(help), kong.UsageOnError())
 
 	out := os.Stdout
 	if cli.Output != "" {