Prepare for 0.4.22 release

What's Changed

Add some clarifications to the library docs by @KodrAus in https://github.com/rust-lang/log/pull/620
Add links to colog crate by @chrivers in https://github.com/rust-lang/log/pull/621
adding line_number test + updating some testing infrastructure by @DIvkov575 in https://github.com/rust-lang/log/pull/619
Clarify the actual set of functions that can race in _racy variants by @KodrAus in https://github.com/rust-lang/log/pull/623
Replace deprecated std::sync::atomic::spin_loop_hint() by @Catamantaloedis in https://github.com/rust-lang/log/pull/625
Check usage of max_level features by @Thomasdezeeuw in https://github.com/rust-lang/log/pull/627
Remove unneeded import by @Thomasdezeeuw in https://github.com/rust-lang/log/pull/628
Loosen orderings for logger initialization in https://github.com/rust-lang/log/pull/632. Originally by @pwoolcoc in https://github.com/rust-lang/log/pull/599
Use Location::caller() for file and line info in https://github.com/rust-lang/log/pull/633. Originally by @Cassy343 in https://github.com/rust-lang/log/pull/520