sql, readme: update timezone support

Via: pingcap/docs-cn#876

README.md

@@ -35,7 +35,7 @@
   + Globalization
     - [Character Set Support](sql/character-set-support.md)
     - [Character Set Configuration](sql/character-set-configuration.md)
-    - [Time Zone](sql/time-zone.md)
+    - [Time Zone Support](sql/time-zone.md)
   + Data Types
     - [Numeric Types](sql/datatype.md#numeric-types)
     - [Date and Time Types](sql/datatype.md#date-and-time-types)

sql/time-zone.md

@@ -1,12 +1,16 @@
 ---
-title: Time Zone
+title: Time Zone Support
 summary: Learn how to set the time zone and its format.
 category: user guide
 ---
 
-# Time Zone
+# Time Zone Support
 
-The time zone in TiDB is decided by the global `time_zone` system variable and the session `time_zone` system variable. The initial value for `time_zone` is 'SYSTEM', which indicates that the server time zone is the same as the system time zone. 
+The time zone in TiDB is decided by the global `time_zone` system variable and the session `time_zone` system variable. The default value of `time_zone` is `SYSTEM`. The actual time zone corresponding to `System` is configured when the TiDB cluster bootstrap is initialized. The detailed logic is as follows:
+
+- Prioritize the use of the `TZ` environment variable.
+- If the `TZ` environment variable fails, extract the time zone from the actual soft link address of `/etc/localtime`.
+- If both of the above methods fail, use `UTC` as the system time zone.
 
 You can use the following statement to set the global server `time_zone` value at runtime: