docs: re-org child_process into async/sync

Cleaner separation between the traditional non-blocking functions and
the new blocking variants.
docs: summarize child_process sync/async sections
docs: child_process improve document linking

Add links and better descriptions where appropriate.
doc: clarify child_process.spawn defaults

Remove an outdated and misleading warning about the behaviour of
spawn when an empty options object is given.

Add test to ensure the behaviour remains.