DOC: Fix compilation error in migration guide
=act:3152 Add doc to LoggingAdapter
=clu:3152 Improve scaladoc of cluster
Merge pull request #1604 from akka/wip-migration-fix-patriknw

DOC: Fix compilation error in migration guide
Merge pull request #1607 from akka/wip-3152-scaladoc2-patriknw

More scaladoc improvements, see #3152
Merge remote-tracking branch upstream/$:{upbranch}