news: add entry for syslog-ng#3754

Signed-off-by: Attila Szakacs <attila.szakacs@oneidentity.com>
Signed-off-by: László Várady <laszlo.varady@protonmail.com>

news/feature-3754.md

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+`disk-buffer`: Added a new option to reliable disk-buffer: `qout-size()`.
+
+This option sets the number of messages that are stored in the memory in addition
+to storing them on disk. The default value is 1000.
+
+This serves performance purposes and offers the same no-message-loss guarantees as
+before.
+
+It can be used to maintain a higher throughput when only a small number of messages
+are waiting in the disk-buffer.