docs: document what happens with extra_check #890

Job	Run time
Check	24s
Test all features	37s
Test feature alpha-async-tokio-rusqlite	38s
Test feature from-directory	44s
Test no features	48s
Clippy (all code)	8s
Clippy (main package)	27s
Code coverage	1m 26s
Documentation	13s
Check with minimal Rust version	53s
Rustfmt	9s
Mutation tests (0)	1m 21s
Mutation tests (1)	1m 22s
Test examples	44s
Mutation tests (2)	1m 16s
Mutation tests (3)	1m 25s
	12m 35s

Provide feedback