[v4.x backport] doc: add documentation for `killed` property of ChildProcess instance

Backport of #14578. (It adds links to preserve old names of links in the 6.x docs. Unneeded in 9.x and forward because those links never existed in those docs.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)