Improve doc comment for GIDSignInButtonStyle

[kamimi01]

I worked on #203

I think to be customizable the display text is better than adding document comments like this, so this is a workaround.

[kamimi01]

@petea Hi team,
Thank you for checking my pull request. As I mentioned, I think to be customizable display text is better than adding doc comments. Do you have a plan to be able to customize it? If you don't, would you mind telling me a reason?
Thank you.