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