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
@BenMorel we actually avoid it. I just felt the nitpicking here unnecessary, but yes, `= null` should not be defined.