refactor: improve CLI value parsing/docs #484

Job	Run time
aarch64-unknown-linux-musl	3m 29s
powerpc64le-unknown-linux-gnu	3m 15s
powerpc-unknown-linux-gnu	3m 20s
armv7-unknown-linux-gnueabihf	3m 22s
aarch64-unknown-linux-gnu	3m 19s
arm-unknown-linux-gnueabihf	3m 19s
arm-unknown-linux-gnueabi	3m 9s
x86_64-unknown-linux-gnu	1m 53s
x86_64-unknown-linux-musl	3m 13s
x86_64-apple-darwin	2m 49s
aarch64-pc-windows-msvc	4m 43s
aarch64-apple-darwin	3m 41s
s390x-unknown-linux-gnu	3m 14s
x86_64-pc-windows-msvc	3m 52s
powerpc64-unknown-linux-gnu	3m 19s
publish	0s
	49m 57s

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