doc: add note to readable stream async iterator docs
While I was working on https://github.com/nodejs/node/pull/18904 I noticed that async iterator in readable streams iterates by the chunks of the size equal to stream highWaterMark
value when reading a file. So I think it worth to mention in docs because at least for me it seemed confusing at first.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
stream, doc