clarify index merge metrics configuration documentation

The documentation – e.g. https://lucene.apache.org/solr/guide/8_6/metrics-reporting.html#index-merge-metrics – currently mentions that "Basic metrics are always collected" but experimentally and from a quick look at the code – e.g. https://github.com/apache/lucene-solr/blob/releases/lucene-solr/8.6.2/solr/core/src/java/org/apache/solr/update/SolrIndexWriter.java#L146 – it appears that a metrics element needs to be configured and that it needs to have either a merge or a mergeDetails element configured as true.