Update truncate token filter to not mention the keyword tokenizer
The advice predates the existence of the keyword field
Closes
=== 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`.
