correct docs for char in rescript about unicode (#579)

* correct docs for char in rescript about unicode

* add a version constraint

Author: bobzhang

misc_docs/syntax/language_char_literal.mdx

@@ -6,7 +6,7 @@ summary: "This is the `char` literal syntax."
 category: "languageconstructs"
 ---
 
-A `char` literal is composed of two **single** quotes. Double quotes are reserved for the `string` type. Note that `char` doesn't support Unicode or UTF-8.
+A `char` literal is composed of two **single** quotes. Double quotes are reserved for the `string` type. Note that `char` is essentially an integer in JS since version 10.0.
 
 ### Example
 
@@ -16,14 +16,20 @@ A `char` literal is composed of two **single** quotes. Double quotes are reserve
 let one = '1';
 let upperCaseA = 'A';
 let lowerCaseA = 'a';
+let unicode ='😀';
+let escape = '\u1111'
 ```
 
+
+
 ```js
 var one = /* '1' */49;
 
 var upperCaseA = /* 'A' */65;
 
 var lowerCaseA = /* 'a' */97;
+var unicode = 128512;
+var escape = 4369;
 ```
 
 </CodeTab>