Document ES|QL categorize limitations (elastic#117892)

* Document ES|QL categorize limitations

* Update x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/grouping/Categorize.java

Co-authored-by: Alexander Spies <[email protected]>

---------

Co-authored-by: Alexander Spies <[email protected]>

Author: jan-elastic

docs/reference/esql/functions/description/categorize.asciidoc

@@ -48,6 +48,12 @@ public class Categorize extends GroupingFunction implements Validatable {
     @FunctionInfo(
         returnType = "keyword",
         description = "Groups text messages into categories of similarly formatted text values.",
+        detailedDescription = """
+            `CATEGORIZE` has the following limitations:
+
+            * can't be used within other expressions
+            * can't be used with multiple groupings
+            * can't be used or referenced within aggregate functions""",
         examples = {
             @Example(
                 file = "docs",