arch: arm: update function documentation for __svc

Fixing the documentation for ARM core __svc function, which was
outdated and present only for the ARMv7-M version.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

arch/arm/core/swap.c

@@ -36,7 +36,7 @@ extern const int _k_neg_eagain;
  * Given that __swap() is called to effect a cooperative context switch,
  * only the caller-saved integer registers need to be saved in the thread of the
  * outgoing thread. This is all performed by the hardware, which stores it in
- * its exception stack frame, created when handling the svc exception.
+ * its exception stack frame, created when handling the __pendsv exception.
  *
  * On ARMv6-M, the intlock key is represented by the PRIMASK register,
  * as BASEPRI is not available.

arch/arm/core/swap_helper.S

@@ -258,6 +258,17 @@ _thread_irq_disabled:
     bx lr
 
 #if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
+
+/**
+ *
+ * @brief Service call handler
+ *
+ * The service call (svc) is used in the following occasions:
+ * - IRQ offloading
+ * - Kernel run-time exceptions
+ *
+ * @return N/A
+ */
 SECTION_FUNC(TEXT, __svc)
     /* Use EXC_RETURN state to find out if stack frame is on the
      * MSP or PSP
@@ -305,16 +316,18 @@ _oops:
     pop {r0, pc}
 
 #elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
+
 /**
  *
  * @brief Service call handler
  *
- * The service call (svc) is only used in __swap() to enter handler mode so we
+ * The service call (svc) is used in the following occasions:
- * can go through the PendSV exception to perform a context switch.
+ * - IRQ offloading
+ * - Kernel run-time exceptions
+ * - System Calls (User mode)
  *
  * @return N/A
  */
-
 SECTION_FUNC(TEXT, __svc)
     tst lr, #0x4    /* did we come from thread mode ? */
     ite eq  /* if zero (equal), came from handler mode */