doc: changelog: fast_pair: add changelog entry for Additional Data fix

Added a changelog entry under the Bluetooth libraries and services
section that records the missing ATT write length validation fix in
the GATT write handler for the Fast Pair Additional Data
characteristic, used by the experimental Personalized Name extension
(CONFIG_BT_FAST_PAIR_PN).

Removed the |no_changes_yet_note| substitution from the section as it
now contains an entry.

Ref: NCSDK-39121

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>

Author: kapi-no

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

@@ -503,7 +503,9 @@ Binary libraries
 Bluetooth libraries and services
 --------------------------------
 
-|no_changes_yet_note|
+* :ref:`bt_fast_pair_readme` library:
+
+  * Fixed missing ATT write length validation in the GATT write handler for the Fast Pair Additional Data characteristic, used by the experimental Personalized Name extension (:kconfig:option:`CONFIG_BT_FAST_PAIR_PN`).
 
 Common Application Framework
 ----------------------------