docs: add warning to maxLength / minLength (#2428)

Author: jasoniangreen

docs/json-schema.md

@@ -280,6 +280,10 @@ The value of the keyword should be a number. The data to be valid should be a mu
 
 ### `maxLength` / `minLength`
 
+::: warning [Grapheme clusters](https://www.unicode.org/reports/tr29/tr29-17.html#Grapheme_Cluster_Boundaries) will count as multiple characters
+Certain charsets have characters that are made up of multiple unicode characters. These "grapheme clusters" are counted as multiple characters.
+:::
+
 The value of the keywords should be a number. The data to be valid should have length satisfying this rule. Unicode pairs are counted as a single character.
 
 **Examples**