Add missing documentation for ILM metrics

[multani]

#513 added the metrics but they were not documented.

I'm not sure about the "cardinality" content ; this is my understanding:

• elasticsearch_ilm_status has an operation_mode label which can have 3 values so I set cardinality to 3
• elasticsearch_ilm_index_status has more labels, in particular index which can be unbounded, so I didn't set any cardinality (since it depends on the number of indices in the cluster).