+act #15383 add strict japi.Option.getOrElse to simplify working with options from Java

(cherry picked from commit 657afbd8591b28c1aef137520479d789bdfae83d)
=doc #3986 Cluster Singleton should not be used with AutoDown
unless you want each partition of the cluster (effectively new clusters),
to spin up their "own" singleton.
Merge pull request #15438 from ktoso/port-clusterSingletonDocs-ktoso

=doc #3986 Cluster Singleton should not be used with AutoDown (for Validation)
Merge pull request #15432 from spray/w/add-japi-Option-getOrElse-backport-to-2.3

+act #15383 add strict japi.Option.getOrElse to simplify working with options from Java (backport)
=per #15429 Rewrite persistence documentation and samples for 2.3.4 changes
Merge pull request #15443 from akka/wip-15429-persistence-docs-patriknw

=per #15429 Rewrite persistence documentation and samples for 2.3.4 changes
+per #15424 Added PersistentView, deprecated View

A PersistentView works the same way as View did previously, except:

* it requires an `peristenceId` (no default is provided)
* messages given to `receive` are NOT wrapped in Persistent()