doc: Update release note for 1.12

Added additional tests for threads and memory domain.
Documented test cases in kernel.

Signed-off-by: Spoorthi K <spoorthi.k@intel.com>

doc/release-notes-1.12.rst

@@ -186,12 +186,14 @@ HALs
 
 Documentation
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-
+* Added description for kernel test cases
 
 Tests and Samples
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 * Added test for POSIX mutex
 * Added Apple iBeacon sample application
+* Enhanced threads test suite
+* Added tests for memory domain
 
 Issue Related Items
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