doc: sort child_process alphabetically

Reorders, with no contextual changes, the child_process documentation
alphabetically.

PR-URL: #3662
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: tflanagan