Retry defaults are not visible in the documentation

[tlathia-figma]

There are a bunch of default values to control the retry behavior including max attempts and delay between attempts. These are not currently visible in the documentation, instead you have to open up the source code. Happy to open a PR to fix this, do you have a preference for where it should be documented? A few options:

• Public constants for the default values, eg. DefaultMaxAttempts = 5
• Include the default values on the relevant Option constructors
• Include the default values in the doc comment for the main Retry function

[bradenaw]

I think I'd prefer (3), like

The defaults are MaxAttempts(5), MinDelay(10 * time.Millisecond), ...