doc: watermark string behavior

Documents that we calculate the highWaterMark value
of streams operating on strings using the number of
UTF-16 code units.

Fixes: nodejs#52818
PR-URL: nodejs#52842
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/stream.md

@@ -317,7 +317,9 @@ buffer.
 The amount of data potentially buffered depends on the `highWaterMark` option
 passed into the stream's constructor. For normal streams, the `highWaterMark`
 option specifies a [total number of bytes][hwm-gotcha]. For streams operating
-in object mode, the `highWaterMark` specifies a total number of objects.
+in object mode, the `highWaterMark` specifies a total number of objects. For
+streams operating on (but not decoding) strings, the `highWaterMark` specifies
+a total number of UTF-16 code units.
 
 Data is buffered in `Readable` streams when the implementation calls
 [`stream.push(chunk)`][stream-push]. If the consumer of the Stream does not