doc,test: clarify --eval syntax for leading '-' scripts #3599

Job	Run time
Build slim tarball	2m 17s
x86_64-linux: with shared libraries	24m 15s
x86_64-darwin: with shared libraries	31m 57s
aarch64-darwin: with shared libraries	19m 47s
aarch64-linux: with shared libraries	20m 31s
	1h 38m 47s

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