doc: matter: Mention new kFactoryReset event in release notes

Mention `kFactoryReset` event in `Matter fork` section.

Signed-off-by: Adrian Gielniewski <adrian.gielniewski@nordicsemi.no>

Author: adigie

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -175,7 +175,10 @@ Matter
 Matter fork
 +++++++++++
 
-|no_changes_yet_note|
+* Added:
+
+  * A new ``kFactoryReset`` event that is posted during factory reset.
+    The application can register a handler and perform additional cleanup.
 
 nRF IEEE 802.15.4 radio driver
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