doc: remove confusing note about child process stdio

It’s not obvious what the paragraph is supposed to say.
In particular, whether and what kind of buffering mechanism
a process uses for its stdio streams does not affect that,
in general, no guarantees can be made about when it consumes data
that was sent to it.

PR-URL: https://github.com/nodejs/node/pull/19552


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>