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

Checklist

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

Affected core subsystem(s)

doc