doc: release note on mempool size in -blocksonly

Adds a release note detailing the new mempool sizing behaviour when
running in blocksonly mode, and instruction on how to override the new
defaults.

doc/release-notes-26471.md

@@ -0,0 +1,13 @@
+Updated settings
+----------------
+
+- Setting `-blocksonly` will now reduce the maximum mempool memory
+  to 5MB (users may still use `-maxmempool` to override). Previously,
+  the default 300MB would be used, leading to unexpected memory usage
+  for users running with `-blocksonly` expecting it to eliminate
+  mempool memory usage.
+
+  As unused mempool memory is shared with dbcache, this also reduces
+  the dbcache size for users running with `-blocksonly`, potentially
+  impacting performance.
+