[TEST] Increase logging on MinDocCountTests.
Convert TermQuery to PrefixQuery if PHRASE_PREFIX is set

We miss to add a single term to a prefix query if the query in only a
single term.

Closes #5551
[DOCS] Clarify nested type documentation
[TEST] Moved wipe* methods, randomIndexTemplate & ensureEstimatedStats from ElasticsearchIntegrationTest to TestCluster

This is the first to make it possible to have a different impl of TestCluster (e.g. based on an external cluster) that has the same methods but a different impl for them (e.g. it might use the REST API to do the same instead of the Java API)

Closes #5542