Tests

Update changelog for 1.42; add `snapshot_kind` explanation #1497

Job	Run time
Run the in-tree version of cargo-insta on ourselves	1m 17s
clippy	17s
Build docs	24s
Test on Latest (ubuntu-latest)	1m 8s
Test on Latest (macos-latest)	57s
Test on Latest (windows-latest)	3m 18s
Check on MSRV	26s
Test latest deps	1m 8s
Test with minimal version resolution	27s
rustfmt	10s
	9m 32s

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