Add note about graceful termination

[mifi]

doesn't work on Windows

[ehmicky]

Hi @mifi and thanks for this PR.

This looks good to me, except you would need to add the new documentation in the TypeScript comments too.

execa/index.d.ts

Line 463 in f067d5f

Same as the original [`child_process#kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal), except if `signal` is `SIGTERM` (the default value) and the child process is not terminated after 5 seconds, force it by sending `SIGKILL`.

The CI is currently failing, but this is due to #563

[ehmicky]

Thanks for the update, this looks good.
Just waiting on #563 to get merged, and @sindresorhus review on this PR.