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
Ok, no worries! I'll keep it in mind next time.
That would be a good point to mention in the [coding standard][http://www.doctrine-project.org/jira/browse/DDC-585)