doc: describe when stdout/err is sync

process.stdout, process.stderr, and console.log() and console.error()
which use the process streams, are usually synchronous. Warn about this,
and clearly describe the conditions under which they are synchronous.

Fix: https://github.com/nodejs/node/issues/10617
PR-URL: https://github.com/nodejs/node/pull/10884


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>