main branch: https://github.com/electron/electron/blob/27fe6cc97f4d40c3a74c58ed1de41c9766437192/docs/api/utility-process.md#childpid

Outdated live version: https://www.electronjs.org/docs/latest/api/utility-process#childpid

The relevant PR was backported to all relevant branches, too: electron/electron#44339