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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines