doc: update subprocess.killed

This commit changes the wording of subprocess.killed to reflect
that a child process was successfully signaled, and not
necessarily terminated.

Fixes: https://github.com/nodejs/node/issues/16747
PR-URL: https://github.com/nodejs/node/pull/16748


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>