Explain why fallback happened when subcommand is not supported, and fix rustdoc #3883

Job	Run time
fmt	23s
cargo-deny	29s
check	36s
clippy (macos-latest)	49s
shellcheck	8s
clippy (ubuntu-latest)	35s
clippy (windows-latest)	1m 48s
test (macos-latest)	2m 26s
test (ubuntu-latest)	2m 3s
test (windows-latest)	4m 9s
generate-matrix	35s
remote	0s
target (${{ matrix.pretty }},${{ matrix.os }})	0s
docker-in-docker	0s
bisect	0s
foreign	0s
podman	0s
publish	0s
conclusion	4s
Coverage	24s
	14m 29s

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