documenting alternative syntax that's supported for DATE_ADD, DATE_SUB

this is related to issue https://github.com/ClickHouse/ClickHouse/issues/25815 

i'm adding examples for:
date_add(date, INTERVAL value unit) and date_sub(date, INTERVAL value unit) 

both are covered with a test from e3c09e9703/tests/queries/0_stateless/02160_special_functions.sql (L33) ; i think it's worth documenting them as the syntax is familiar to MySQL users [ https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-add ]

docs/en/sql-reference/functions/date-time-functions.md

@@ -1902,6 +1902,12 @@ If the addition results in a value outside the bounds of the data type, the resu
 date_add(unit, value, date)
 ```
 
+Alternative syntax:
+
+``` sql
+date_add(date, INTERVAL value unit)
+```
+
 Aliases: `dateAdd`, `DATE_ADD`.
 
 **Arguments**
@@ -1941,6 +1947,20 @@ Result:
 └───────────────────────────────────────────────┘
 ```
 
+```sql
+SELECT date_add(toDate('2018-01-01'), INTERVAL 3 YEAR);
+```
+
+Result:
+
+```text
+┌─plus(toDate('2018-01-01'), toIntervalYear(3))─┐
+│                                    2021-01-01 │
+└───────────────────────────────────────────────┘
+```
+
+
+
 **See Also**
 
 - [addDate](#addDate)
@@ -1957,6 +1977,13 @@ If the subtraction results in a value outside the bounds of the data type, the r
 date_sub(unit, value, date)
 ```
 
+Alternative syntax:
+
+``` sql
+date_sub(date, INTERVAL value unit)
+```
+
+
 Aliases: `dateSub`, `DATE_SUB`.
 
 **Arguments**
@@ -1997,6 +2024,19 @@ Result:
 └────────────────────────────────────────────────┘
 ```
 
+``` sql
+SELECT date_sub(toDate('2018-01-01'), INTERVAL 3 YEAR);
+```
+
+Result:
+
+``` text
+┌─minus(toDate('2018-01-01'), toIntervalYear(3))─┐
+│                                     2015-01-01 │
+└────────────────────────────────────────────────┘
+```
+
+
 **See Also**
 
 - [subDate](#subDate)