[radio] clarify mTimestamp meaning in comments (#8894)

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (303)
+#define OPENTHREAD_API_VERSION (304)
 
 /**
  * @addtogroup api-instance

include/openthread/platform/radio.h

@@ -334,8 +334,7 @@ typedef struct otRadioFrame
             /**
              * The timestamp when the frame was received in microseconds.
              *
-             * The value SHALL be the time when the SFD was received when TIME_SYNC or CSL is enabled.
-             * Otherwise, the time when the MAC frame was fully received is also acceptable.
+             * The value SHALL be the time when the SFD was received.
              *
              */
             uint64_t mTimestamp;

src/core/mac/mac_frame.hpp

@@ -1206,8 +1206,9 @@ public:
 
     /**
      * This method returns the timestamp when the frame was received.
+     * The timestamp marks the frame detection time: the end of the last symbol of SFD.
      *
-     * @returns The timestamp when the frame was received, in microseconds.
+     * @returns The timestamp when the frame SFD was received, in microseconds.
      *
      */
     const uint64_t &GetTimestamp(void) const { return mInfo.mRxInfo.mTimestamp; }