Add explanation for auto_random ID allocation (#16113)

auto-random.md

@@ -128,6 +128,10 @@ The output is as follows:
 1 row in set (0.00 sec)
 ```
 
+## Implicit allocation rules of IDs
+
+TiDB implicitly allocates values to `AUTO_RANDOM` columns similarly to `AUTO_INCREMENT` columns. They are also controlled by the session-level system variables [`auto_increment_increment`](/system-variables.md#auto_increment_increment) and [`auto_increment_offset`](/system-variables.md#auto_increment_offset). The auto-increment bits (ID) of implicitly allocated values conform to the equation `(ID - auto_increment_offset) % auto_increment_increment == 0`.
+
 ## Restrictions
 
 Pay attention to the following restrictions when you use `AUTO_RANDOM`:

system-variables.md

@@ -257,7 +257,7 @@ For more information about the `SET_VAR` hint, see [SET_VAR](/optimizer-hints.md
 - Type: Integer
 - Default value: `1`
 - Range: `[1, 65535]`
-- Controls the step size of `AUTO_INCREMENT` values to be allocated to a column. It is often used in combination with `auto_increment_offset`.
+- Controls the step size of `AUTO_INCREMENT` values to be allocated to a column, and allocation rules for `AUTO_RANDOM` IDs. It is often used in combination with [`auto_increment_offset`](#auto_increment_offset).
 
 ### auto_increment_offset
 
@@ -267,7 +267,7 @@ For more information about the `SET_VAR` hint, see [SET_VAR](/optimizer-hints.md
 - Type: Integer
 - Default value: `1`
 - Range: `[1, 65535]`
-- Controls the initial offset of `AUTO_INCREMENT` values to be allocated to a column. This setting is often used in combination with `auto_increment_increment`. For example:
+- Controls the initial offset of `AUTO_INCREMENT` values to be allocated to a column, and allocation rules for `AUTO_RANDOM` IDs. This setting is often used in combination with [`auto_increment_increment`](#auto_increment_increment). For example:
 
 ```sql
 mysql> CREATE TABLE t1 (a int not null primary key auto_increment);