Fix document comment of GoogleSignIn (product "GoogleSignIn")

[kamimi01]

Description

I've realized that there is a difference between the display text of ".standard" and ".wide" when we use "GIDSignInButtonStyle" property.
But there is no document comments in GIDSignInButton.h about it.
I think that should be written in the source code or the display text should be able to customize

• standard: "Sign in"
• wide: "Sign in with Google"

standard	wide

[w-goog]

Thanks for your contribution, @kamimi01! Regarding text customization - that's not a change the iOS library is planning to support at this time. If there's a gap in our localization, please let us know by reporting an issue here :)

Or, if you find one of the default buttons otherwise supported in the brand guidelines has specific issues on iOS, similarly, please open an issue.