Improve docs regarding integrations with other frameworks or libraries

[ihoro]

For example, Kyua supports ATF tests: a test program can contain multiple test cases, Kyua understands some metadata coming from the ATF side, and so on. For now, according to the #258 it's documented right in the kyuafile.5 man page for convenience, but sooner or later we may want to aggregate such information in separate man page(s). See the original discussion here: https://reviews.freebsd.org/D48190.