doc: document watermark string behavior

[benjamingr]

Bikeshedding/doc suggestions (should we edit it in other places like (readableLength/writableLength?) welcome

ping @lpinca @nodejs/streams

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

Fixes: https://github.com/nodejs/node/issues/52818

[lpinca]

I'm not sure if the "but not decoding" bit is sufficient, it might be a bit too generic for someone not familiar with the implementation, but I can't think of anything better to suggest.

[redyetidev]

(3[+1] approving reviews; no failed tests)

[aduh95]

Landed in 761d2ce43cc5c996f4bdc5b873384eb4f2346488