doc: release: add notes on RISC-V's stack unwinding

Add an entry in the RISC-V arch section about the
implementation of frame-pointer based stack unwinding.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
diff --git a/doc/releases/release-notes-3.7.rst b/doc/releases/release-notes-3.7.rst
index aa6d4d2..9a31f7f 100644
--- a/doc/releases/release-notes-3.7.rst
+++ b/doc/releases/release-notes-3.7.rst
@@ -35,6 +35,10 @@
 
 * ARM
 
+* RISC-V
+
+  * Implemented frame-pointer based stack unwinding.
+
 * Xtensa
 
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