doc: Mention dbcache increase in release notes

Author: laanwj

doc/release-notes.md

@@ -41,9 +41,21 @@ report issues about Windows XP to the issue tracker.
 Notable changes
 ===============
 
-Example item
-----------------
+Database cache memory increased
+--------------------------------
+
+As a result of growth of the UTXO set, performance with the prior default
+database cache of 100 MiB has suffered.
+For this reason the default was changed to 300 MiB in this release.
+
+For nodes on low-memory systems, the database cache can be changed back to
+100 MiB (or to another value) by either:
+
+- Adding `dbcache=100` in bitcoin.conf
+- Changing it in the GUI under `Options → Size of database cache`
 
+Note that the database cache setting has the most performance impact
+during initial sync of a node, and when catching up after downtime.
 
 bitcoin-cli: arguments privacy
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