Add release notes item

Signed-off-by: Dirkjan Bussink <[email protected]>

changelog/20.0/20.0.0/summary.md

@@ -6,6 +6,7 @@
   - **[Breaking changes](#breaking-changes)**
     - [`shutdown_grace_period` Default Change](#shutdown-grace-period-default)
     - [New `unmanaged` Flag and `disable_active_reparents` deprecation](#unmanaged-flag)
+    - [`mysqlctld` `onterm-timeout` Default Change](#mysqlctld-onterm-timeout)
   - **[Query Compatibility](#query-compatibility)**
     - [Vindex Hints](#vindex-hints)
     - [Update with Limit Support](#update-limit)
@@ -38,6 +39,12 @@ New flag `--unmanaged` has been introduced in this release to make it easier to
 
 Starting this release, all unmanaged tablets should specify this flag.
 
+#### <a id="mysqlctld-onterm-timeout"/>`mysqlctld` `onterm_timeout` Default Change
+
+The `--onterm_timeout` flag default value has changed for `mysqlctld`. It now is by default long enough to be able to wait for the default `--shutdown-wait-time` when shutting down on a `TERM` signal. 
+
+This is necessary since otherwise MySQL would never shut down cleanly with the old defaults, since `mysqlctld` would shut down already after 10 seconds by default.
+
 ### <a id="query-compatibility"/>Query Compatibility
 
 #### <a id="vindex-hints"/> Vindex Hints