Update handover strategy doc about shared folder and "handover complete" file

[ramirogm]

I'm working with the handover update strategy on product-os/jellyfish#8836
While trying to save the "handover complete file", I get an error saving to /data/resin-kill-me , in fact /data is not mounted.
What works is saving to /tmp/balena/handover-complete and I think saving to /tmp/resin/resin-kill-me would also work.

See https://github.com/balena-os/balena-supervisor/blob/6d6f09eb59fde7e3a2eb10de0d746b44516249f7/src/compose/service.ts#L936

Related to #2342 @vipulgupta2048

[vipulgupta2048]

Thanks this is good @ramirogm, please go ahead and make a PR to add these changes in. That PR can be used to confirm if this is actually the behavior we want or not.