1579 cleanup

This wording makes it sound like a new deprecation.  Really, this was deprecated in 2.99.0 but we forgot to issue a proper deprecation warning in 2.99.0.  I'd categorize this as a bug fix.  Something like:

```
Bug Fixes:

* Add missing deprecation warning for when `RSpec::Core::Runner` is used multiple times
  in the same process. In 2.x RSpec's global state was automatically cleared between runs
  but in 3.0 you need to call `RSpec.reset` manually in these situations. (Sam Phippen, #1587)
```