s/any kind of/any cross-devices/

Thanks to Bruno Racineux for reporting this.
Misc rewriting. Add an explicit MUST.
Use Promise<void> instead of Promise<undefined>.
Merge pull request #54 from mounirlamouri/misc_rewriting

Misc rewriting. Add an explicit MUST.
Merge pull request #55 from mounirlamouri/promise_void

Use Promise<void> instead of Promise<undefined>.