doc: stdout/stderr can block when directed to file

Update the documentation for `process.stdout` and `process.stdout` to
clarify that writes can block when stdio is redirected to a file.  In
all other cases, it's non-blocking.

PR-URL: https://github.com/nodejs/node/pull/3170


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>