[DOCS] Add categorization limitation (elastic/x-pack-elasticsearch#3022)

Original commit: elastic/x-pack-elasticsearch@387d7cf939

docs/en/ml/categories.asciidoc

@@ -11,10 +11,15 @@ example:
 //NOTCONSOLE
 
 You can use {ml} to observe the static parts of the message, cluster similar
-messages together, and classify them into message categories. The {ml} model
-learns what volume and pattern is normal for each category over time. You can
-then detect anomalies and surface rare events or unusual types of messages by
-using count or rare functions. For example:
+messages together, and classify them into message categories.
+
+NOTE: Categorization uses English tokenization rules and dictionary words in
+order to identify log message categories. As such, only English language log
+messages are supported.
+
+The {ml} model learns what volume and pattern is normal for each category over
+time. You can then detect anomalies and surface rare events or unusual types of
+messages by using count or rare functions. For example:
 
 //Obtained from it_ops_new_app_logs.sh
 [source,js]

docs/en/ml/limitations.asciidoc

@@ -4,6 +4,13 @@
 The following limitations and known problems apply to the {version} release of
 {xpack}:
 
+[float]
+=== Categorization uses English tokenization rules and dictionary words
+//See x-pack-elasticsearch/#3021
+Categorization identifies static parts of unstructured logs and groups similar
+messages together. This is currently supported only for English language log
+messages.
+
 [float]
 === Pop-ups must be enabled in browsers
 //See x-pack-elasticsearch/#844