core: add note about lag in buflist in release notes

ReleaseNotes.adoc

@@ -22,6 +22,22 @@ The following relay options have been renamed:
 * relay.color.status_waiting_auth -> relay.color.status_authenticating
 * relay.weechat.commands -> relay.network.commands (new default value: `*,!quit`)
 
+[[v4.3.0_lag_in_buflist]]
+=== Lag in buflist
+
+The lag is now stored in all IRC buffers: server (like it always has been),
+channels and private buffers.
+
+Consequently, if you use `${format_lag}` in buflist options, this lag will be
+displayed on server and all channels and private buffers.
+
+If you want to display the lag only on server buffer in buflist, you can use
+such format:
+
+----
+${if:${type}==server?${format_lag}}
+----
+
 [[v4.3.0_irc_color_channel_modes]]
 === Color of IRC channel modes