Improved docs

[Genarito]

Hi! First of all, thank you for this library. It is my first time using it, and thanks to the IDE I discovered that there was an image parameter in the provisioning_uri method. Also, looking at the source code I discovered that it consisted of a URL.

In this PR, the image parameter is defined explicitly in all the methods involved and added some documentation for it. I also added some missing documentation for methods such as byte_secret or provisioning_uri (from the TOTP class).

I also considered it was good to document that the digits parameter in the Steam class is discarded and hardcoded to 10. So I added this clarification and changed the default value to make it more intuitive to the naked eye.

Finally, I standardized a little bit, adding the missing dots in sentences, I documented a :return and corrected a typo.

I remain pending if you consider that other changes should be made. Greetings!