docs: An interface can include interfaces only and not modules

[tk0miya]

I found an interface can include interfaces only and not modules. Is my understanding correct?
If true, it's better to fix the syntax document.

Example:

# sig/example.rbs
module Foo
end

interface _Bar
  include Foo
end

bundle exec rbs -I sig validate                                                                                                                        ~/work/tmp/steep [main]
bundler: failed to load command: rbs (/Users/tkomiya/.dotfiles/_rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/bin/rbs)
/Users/tkomiya/.dotfiles/_rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rbs-3.5.3/lib/rbs/parser_aux.rb:20:in `_parse_signature': sig/example.rbs:5:10...5:13: Syntax error: expected one of interface name, token=`Foo` (tUIDENT) (RBS::ParsingError)

    include Foo
            ^^^

	from /Users/tkomiya/.dotfiles/_rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rbs-3.5.3/lib/rbs/parser_aux.rb:20:in `parse_signature'
	from /Users/tkomiya/.dotfiles/_rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rbs-3.5.3/lib/rbs/environment_loader.rb:161:in `block (2 levels) in each_signature'
(snip)

[soutaro]

Thanks!