Docs: Update SQL example for current_time() and current_date(). (apache#18200)

## Which issue does this PR close?
- Closes apache#18199 
## What changes are included in this PR?
- Added a SQL example in scalar functions md to demonstrate setting
execution time zone (optional) for current_time() and current_date().
## Are these changes tested?
## Are there any user-facing changes?

Author: codetyri0n

datafusion/functions/src/datetime/current_date.rs

@@ -36,7 +36,9 @@ Returns the current date in the session time zone.
 
 The `current_date()` return value is determined at query time and will return the same date, no matter when in the query plan the function executes.
 "#,
-    syntax_example = "current_date()"
+    syntax_example = r#"current_date()
+    (optional) SET datafusion.execution.time_zone = '+00:00';
+    SELECT current_date();"#
 )]
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub struct CurrentDateFunc {

datafusion/functions/src/datetime/current_time.rs

@@ -38,7 +38,9 @@ The `current_time()` return value is determined at query time and will return th
 
 The session time zone can be set using the statement 'SET datafusion.execution.time_zone = desired time zone'. The time zone can be a value like +00:00, 'Europe/London' etc.
 "#,
-    syntax_example = "current_time()"
+    syntax_example = r#"current_time()
+    (optional) SET datafusion.execution.time_zone = '+00:00';
+    SELECT current_time();"#
 )]
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub struct CurrentTimeFunc {

docs/source/user-guide/sql/scalar_functions.md

@@ -2409,6 +2409,8 @@ The `current_date()` return value is determined at query time and will return th
 
 ```sql
 current_date()
+    (optional) SET datafusion.execution.time_zone = '+00:00';
+    SELECT current_date();
 ```
 
 #### Aliases
@@ -2425,6 +2427,8 @@ The session time zone can be set using the statement 'SET datafusion.execution.t
 
 ```sql
 current_time()
+    (optional) SET datafusion.execution.time_zone = '+00:00';
+    SELECT current_time();
 ```
 
 ### `current_timestamp`