doc: document bytes to chars after setEncoding

This commit documents and edge-case behavior in readable streams.
It is expected that non-object streams are measured in bytes
against the highWaterMark. However, it was discovered in issue
thereafter begin to measure the buffer's length in characters.

PR-URL: https://github.com/nodejs/node/pull/13442
Refs: https://github.com/nodejs/node/issues/6798


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>