[log] add region code log (#7740)

This commit adds log to record the region code to help developers know
which region code is currently in use from log.

src/lib/spinel/radio_spinel_impl.hpp

@@ -2409,7 +2409,22 @@ exit:
 template <typename InterfaceType, typename ProcessContextType>
 otError RadioSpinel<InterfaceType, ProcessContextType>::SetRadioRegion(uint16_t aRegionCode)
 {
-    return Set(SPINEL_PROP_PHY_REGION_CODE, SPINEL_DATATYPE_UINT16_S, aRegionCode);
+    otError error;
+
+    error = Set(SPINEL_PROP_PHY_REGION_CODE, SPINEL_DATATYPE_UINT16_S, aRegionCode);
+
+    if (error == OT_ERROR_NONE)
+    {
+        otLogNotePlat("Set region code \"%c%c\" successfully", static_cast<char>(aRegionCode >> 8),
+                      static_cast<char>(aRegionCode));
+    }
+    else
+    {
+        otLogWarnPlat("Failed to set region code \"%c%c\": %s", static_cast<char>(aRegionCode >> 8),
+                      static_cast<char>(aRegionCode), otThreadErrorToString(error));
+    }
+
+    return error;
 }
 
 template <typename InterfaceType, typename ProcessContextType>

src/posix/platform/radio_url.cpp

@@ -89,7 +89,8 @@ const char *otSysGetRadioUrlHelpString(void)
 #endif
 
     return "RadioURL:\n" OT_RADIO_URL_HELP_BUS OT_RADIO_URL_HELP_MAX_POWER_TABLE
-           "    region[=region-code]          Set the radio's region code.\n"
+           "    region[=region-code]          Set the radio's region code. The region code must be an\n"
+           "                                  ISO 3166 alpha-2 code.\n"
            "    cca-threshold[=dbm]           Set the radio's CCA ED threshold in dBm measured at antenna connector.\n"
            "    enable-coex[=1|0]             If not specified, RCP coex operates with its default configuration.\n"
            "                                  Disable coex with 0, and enable it with other values.\n"