core-system/newlib: updated docs to reflect that tzset() needs to be called before localtime/mktime()

According to the posix standard localtime/mktime should use timezone information as though localtime/mktime() calls tzset(),
but this is not the case in older versions of Newlib.

Closes https://github.com/espressif/esp-idf/issues/11455

docs/en/api-reference/system/system_time.rst

@@ -75,6 +75,9 @@ If you need to obtain time with one second resolution, use the following method:
     time(&now);
     // Set timezone to China Standard Time
     setenv("TZ", "CST-8", 1);
+
+    /* According to the posix standard localtime/mktime should use timezone information as though localtime/mktime() calls tzset(),
+       but this is not the case in older versions of Newlib. We manually call tzset to ensure the timezone info is updated*/
     tzset();
 
     localtime_r(&now, &timeinfo);