Provide doc input on cluster id option

Once the new option is added, we need documentation. The basic usage is straight forward, just add the option the the respective remote write config and every metric send through that config will have the _id label with the cluster id as its label.

A more advanced use case is if a user wants to change either the label name (_id) or value. This can be done through providing the right relabel config entries. Lets document the following two cases:

• Change the label name, for example to cluster_id
• Use the label_value in another label and drop the added _id label