doc: updated doc for Zephyr release 1.9.

Updated doc for rel 1.9 for :-
1. TICKLESS Kernel support for nrf RTC timer.
2. Added test cases for stress test round robin scheduling and priority
premptive scheduling.

Signed-off-by: Youvedeep Singh <[email protected]>

Author: youvedeep-singh

doc/release-notes-1.9.rst

@@ -50,6 +50,7 @@ Drivers and Sensors
 
 * KW40Z IEEE 802.15.4 radio driver support added
 * APDS9960 sensor driver added
+* Added TICKLESS KERNEL support for nrf RTC Timer.
 
 Networking
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@@ -125,7 +126,9 @@ Documentation
 Tests and Samples
 *****************
 
-* change description
+* Added test Case to stress test round robin scheduling in schedule_api test.
+* Added test case to stress test priority scheduling in scheduling_api_test.
+
 
 JIRA Related Items
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