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
FYI, I've added a note to [my draft](https://gist.github.com/BenMorel/4676670) mentioned in the link above.
It would be nice if we could move forward with this standard, one day.