docs: release-notes: Add details on some fixed issues

Adds details about some fixed serial recovery issues

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

docs/release-notes.d/single-slot-fixes.md

@@ -0,0 +1,5 @@
+ - Fixed wrong define specifying 2 slots in single loader mode
+   instead of just 1
+ - Fixed wrong slot ID in hook calls from serial recovery.
+ - Fixed issues with serial recovery not building/not
+   working/faulting.