Update documentation on RetryPlugin

[Nyholm]

The plugin has some new parameters that should be documented.

[kozlice]

It's also confusing that parameter retry is called retries in docs. I had to check the actual configurator's source code to find out why it wasn't working. The comment there says // TODO: should be called retries for consistency with the class :)

[dbu]

uh. then we should probably support both, but report a deprecation warning when using retry.