docs: document what happens with extra_check #891

Job	Run time
Check	22s
Test no features	1m 2s
Rustfmt	4s
Documentation	12s
Check with minimal Rust version	50s
Mutation tests (0)	1m 17s
Test all features	44s
Test feature alpha-async-tokio-rusqlite	47s
Mutation tests (1)	1m 7s
Test examples	48s
Test feature from-directory	59s
Mutation tests (2)	1m 5s
Clippy (all code)	12s
Mutation tests (3)	1m 22s
Clippy (main package)	34s
Code coverage	2m 0s
	13m 25s

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