Fixed CacheProvider documentation

Fixed the following:

- `$namespaceVersion` can only be `integer` or `null`, not `string`
- `getNamespaceVersion()` can only return `integer`, not `string`
- `bool` was used in once place, instead of `boolean` everywhere else
Done in https://github.com/doctrine/cache/commit/95818515f9db846b152e3df11eb40ca27e1313f7