Log message on attempted parallel blockchain download; javadoc edit.

Changes I made on my repo while diagnosing a problem.  The log message written during an attempted parallel download indicates it's the `blockChainDownloadLocked()` method writing the log message, but not which method invoked `blockChainDownloadLocked()`.  I changed that.

Also I had difficulty understanding the javadoc description of `AbstractBlockChain.getOrphanRoot()`.  I made some slight modification to that text and its formatting.