Windows not mentioned in multi-os.md

[walles]

https://github.com/travis-ci/docs-travis-ci-com/blob/master/user/multi-os.md:

Travis CI can test on Linux and macOS.

But then there's https://github.com/travis-ci/docs-travis-ci-com/blob/master/user/reference/windows.md describing...

the Travis Windows CI environment

Would you please consider updating multi-os.md to mention Windows as well?

[native-api]

It's officially still "in early stages" and many things available for other OSes aren't supported.

[stale]

Thanks for contributing to this issue. As it has been 90 days since the last activity, we are automatically closing the issue. This is often because the request was already solved in some way and it just wasn't updated or it's no longer applicable. If that's not the case, please do feel free to either reopen this issue or open a new one. We'll gladly take a look again! You can read more here: https://blog.travis-ci.com/2018-03-09-closing-old-issues