docs: add map desc in create table (#3912)

Author: aceforeverd

docs/en/openmldb_sql/data_types/composite_types.md

@@ -28,3 +28,5 @@ select map (1, "12", 2, "100")[2]
 1. Generally not recommended to store a map value with too much key-value pairs, since it's a row-based storage model.
 2. Map data type can not used as the key or ts column of table index, queries can not be optimized based on specific key value inside a map column neither.
 3. Query a key-value in a map takes `O(n)` complexity at most.
+4. Currently, it is not allowed to output a map type value from a SQL query, however you can access information about the map value using map-related expressions. For example, you may use `[]` operator over a `map<int, int>` type to extract value of specific key.
+

docs/en/openmldb_sql/ddl/CREATE_TABLE_STATEMENT.md

@@ -50,15 +50,16 @@ ColumnName ::=
     Identifier ( '.' Identifier ( '.' Identifier )? )?      
 
 ColumnType ::=
-						'INT' | 'INT32'
-						|'SMALLINT' | 'INT16'
-						|'BIGINT' | 'INT64'
-						|'FLOAT'
-						|'DOUBLE'
-						|'TIMESTAMP'
-						|'DATE'
-						|'BOOL'
-						|'STRING' | 'VARCHAR'
+    'INT' | 'INT32'
+    |'SMALLINT' | 'INT16'
+    |'BIGINT' | 'INT64'
+    |'FLOAT'
+    |'DOUBLE'
+    |'TIMESTAMP'
+    |'DATE'
+    |'BOOL'
+    |'STRING' | 'VARCHAR'
+    | 'MAP' '<' ColumnType ',' ColumnType '>'
 
 ColumnOptionList ::= 
     ColumnOption*	

docs/zh/openmldb_sql/data_types/composite_types.md

@@ -27,3 +27,4 @@ select map (1, "12", 2, "100")[2]
 1. 由于采用行存储形式，不建议表的 MAP 类型存储 key-value pair 特别多的情况，否则可能导致性能问题。
 2. map 数据类型不支持作为索引的 key 或 ts 列，无法对 map 列特定 key 做查询优化。 
 3. map key-value 查询最多消耗 `O(n)` 复杂度
+4. 目前暂未支持查询结果直接输出 map 类型，但可以用 map 相关的表达式得到关于 map 值的基本类型结果，例如用 `[]` 获取 `map<int,int>` 中特定 key 的 value。

docs/zh/openmldb_sql/ddl/CREATE_TABLE_STATEMENT.md

@@ -50,15 +50,16 @@ ColumnName ::=
     Identifier ( '.' Identifier ( '.' Identifier )? )?      
 
 ColumnType ::=
-						'INT' | 'INT32'
-						|'SMALLINT' | 'INT16'
-						|'BIGINT' | 'INT64'
-						|'FLOAT'
-						|'DOUBLE'
-						|'TIMESTAMP'
-						|'DATE'
-						|'BOOL'
-						|'STRING' | 'VARCHAR'
+    'INT' | 'INT32'
+    |'SMALLINT' | 'INT16'
+    |'BIGINT' | 'INT64'
+    |'FLOAT'
+    |'DOUBLE'
+    |'TIMESTAMP'
+    |'DATE'
+    |'BOOL'
+    |'STRING' | 'VARCHAR'
+    | 'MAP' '<' ColumnType ',' ColumnType '>'
 
 ColumnOptionList ::= 
     ColumnOption*	
@@ -511,4 +512,4 @@ create table t1 (col0 string, col1 int) options (DISTRIBUTION=[('127.0.0.1:30921
 
 [CREATE DATABASE](../ddl/CREATE_DATABASE_STATEMENT.md)
 
-[USE DATABASE](../ddl/USE_DATABASE_STATEMENT.md)
+[USE DATABASE](../ddl/USE_DATABASE_STATEMENT.md)