Rename `doc_cfg` to `docsrs` and use `doc_auto_cfg` (#1450) #839

Job	Run time
Check doc	1m 3s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	50s
test-miri	3m 47s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 15s
test (macos-latest, x86_64-apple-darwin, stable)	46s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 48s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 29s
test-ios	22s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 47s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 45s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 35s
test-no-std	14s
	17m 41s

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