add note for utf-8 char length in vid (#1871)

We have this note in cn docs already.

related issue: vesoft-inc/nebula-importer#257

docs-2.0/3.ngql-guide/9.space-statements/1.create-space.md

@@ -25,7 +25,7 @@ CREATE SPACE [IF NOT EXISTS] <graph_space_name> (
 |`<graph_space_name>`|Uniquely identifies a graph space in a NebulaGraph instance. The name of the graph space starts with a letter, supports 1 to 4 bytes UTF-8 encoded characters, such as English letters (case-sensitive), digits, and Chinese characters, but does not support special characters except underscores. To use special characters or reserved keywords as identifiers, quote them with backticks. For more information, see [Keywords and reserved words](../../3.ngql-guide/1.nGQL-overview/keywords-and-reserved-words.md).|
 |`partition_num`|Specifies the number of partitions in each replica. The suggested value is 20 times (2 times for HDD) the number of the hard disks in the cluster. For example, if you have three hard disks in the cluster, we recommend that you set 60 partitions. The default value is 100.|
 |`replica_factor`|Specifies the number of replicas in the cluster. The suggested number is 3 in a production environment and 1 in a test environment. The replica number must be an **odd number** for the need of quorum-based voting. The default value is 1.|
-|`vid_type`|A required parameter. Specifies the VID type in a graph space. Available values are `FIXED_STRING(N)` and `INT64`. `INT` equals to `INT64`. <br>``FIXED_STRING(<N>)` specifies the VID as a string, while `INT64` specifies it as an integer. `N` represents the maximum length of the VIDs. If you set a VID that is longer than `N` bytes, NebulaGraph throws an error.|
+|`vid_type`|A required parameter. Specifies the VID type in a graph space. Available values are `FIXED_STRING(N)` and `INT64`. `INT` equals to `INT64`. <br>``FIXED_STRING(<N>)` specifies the VID as a string, while `INT64` specifies it as an integer. `N` represents the maximum length of the VIDs. If you set a VID that is longer than `N` bytes, NebulaGraph throws an error. Note, for UTF-8 chars, the length may vary in different cases, i.e. a UTF-8 Chinese char is 3 byte, this means 11 Chinese chars(length-33) will exeed a FIXED_STRING(32) vid defination|
 |`COMMENT`|The remarks of the graph space. The maximum length is 256 bytes. By default, there is no comments on a space.|
 
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