[mysql] Add note for MySQL TINYINT(1) type to Flink SQL Boolean type

This closes #2030.

flink-connector-mysql-cdc/src/main/java/com/ververica/cdc/connectors/mysql/schema/MySqlTypeUtils.java

@@ -106,6 +106,10 @@ public class MySqlTypeUtils {
         String typeName = column.typeName();
         switch (typeName) {
             case TINYINT:
+                // MySQL haven't boolean type, it uses tinyint(1) to represents boolean type
+                // user should not use tinyint(1) to store number although jdbc url parameter
+                // tinyInt1isBit=false can help change the return value, it's not a general way
+                // btw: mybatis and mysql-connector-java map tinyint(1) to boolean by default
                 return column.length() == 1 ? DataTypes.BOOLEAN() : DataTypes.TINYINT();
             case TINYINT_UNSIGNED:
             case TINYINT_UNSIGNED_ZEROFILL: