Update truncate token filter to not mention the keyword tokenizer

The advice predates the existence of the keyword field

Closes #22650

docs/reference/analysis/tokenfilters/truncate-tokenfilter.asciidoc

@@ -2,9 +2,7 @@
 === Truncate Token Filter
 
 The `truncate` token filter can be used to truncate tokens into a
-specific length. This can come in handy with keyword (single token)
-based mapped fields that are used for sorting in order to reduce memory
-usage.
+specific length.
 
 It accepts a `length` parameter which control the number of characters
 to truncate to, defaults to `10`.