[SPARK-48976][SQL][DOCS] Improve the docs related to variable

### What changes were proposed in this pull request?
The pr aims to improve the docs related to `variable`, includes:
- `docs/sql-ref-syntax-aux-set-var.md`, show the `primitive` error messages.
- `docs/sql-ref-syntax-ddl-declare-variable.md`, add usage of `DECLARE OR REPLACE`.
- `docs/sql-ref-syntax-ddl-drop-variable.md`, show the `primitive` error messages and fix `typo`.

### Why are the changes needed?
Only improve docs.

### Does this PR introduce _any_ user-facing change?
Yes, make end-user docs clearer.

### How was this patch tested?
Manually test.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#47460 from panbingkun/SPARK-48976.

Authored-by: panbingkun <panbingkun@baidu.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

docs/sql-ref-syntax-aux-set-var.md

@@ -85,7 +85,7 @@ SELECT var1, var2;
 
 -- Too many rows
 SET VAR (var1, var2) = (SELECT c1, CAST(c1 AS STRING) FROM VALUES(1), (2) AS t(c1));
-Error: ROW_SUBQUERY_TOO_MANY_ROWS
+[ROW_SUBQUERY_TOO_MANY_ROWS] More than one row returned by a subquery used as a row. SQLSTATE: 21000
 
 -- No rows
 SET VAR (var1, var2) = (SELECT c1, CAST(c1 AS STRING) FROM VALUES(1), (2) AS t(c1) WHERE 1=0);

docs/sql-ref-syntax-ddl-declare-variable.md

@@ -72,6 +72,17 @@ DECLARE [ OR REPLACE ] [ VARIABLE ]
 -- The dense form of declaring a variable with default
 DECLARE five = 5;
 
+-- Declare a defined variable
+DECLARE five = 55;
+[VARIABLE_ALREADY_EXISTS] Cannot create the variable `system`.`session`.`five` because it already exists.
+Choose a different name, or drop or replace the existing variable. SQLSTATE: 42723
+
+-- Use `DECLARE OR REPLACE` to declare a defined variable
+DECLARE OR REPLACE five = 55;
+
+-- Explicitly declare the default value of a variable using the keyword `DEFAULT`
+DECLARE VARIABLE size DEFAULT 6;
+
 -- STRING variable initialialized to `NULL`
 DECLARE some_var STRING;
 ```

docs/sql-ref-syntax-ddl-drop-variable.md

@@ -54,10 +54,11 @@ DROP TEMPORARY VARIABLE var1;
 
 -- Try to drop temporary variable which is not present
 DROP TEMPORARY VARIABLE var1;
-Error: VARIABLE_NOT_FOUND
-The variable `system`.`session`.`var1` cannot be found.
+[VARIABLE_NOT_FOUND] The variable `system`.`session`.`var1` cannot be found. Verify the spelling and correctness of the schema and catalog.
+If you did not qualify the name with a schema and catalog, verify the current_schema() output, or qualify the name with the correct schema and catalog.
+To tolerate the error on drop use DROP VARIABLE IF EXISTS. SQLSTATE: 42883
 
--- Drop temporart variable if it exists
+-- Drop temporary variable if it exists
 DROP TEMPORARY VARIABLE IF EXISTS var1;
 ```