commit() only guarantees documents are persisted to the Directory; an
already-open IndexReader only picks up the change according to its
ReloadPolicy. With the default OnCommitWithDelay, the reload happens
asynchronously on a background thread and is not guaranteed to be done
by the time commit() returns, on any Directory implementation
(including RamDirectory), so a search immediately after commit() can
still miss the documents just committed.
Document this explicitly on ReloadPolicy::OnCommitWithDelay,
IndexReader::reload(), and IndexWriter::commit(), and add a runnable
example showing the ReloadPolicy::Manual + explicit reader.reload()
pattern for deterministic read-your-writes.
See #1824
