doc: align Buffer.concat documentation with behavior #2426

Job	Run time
Build slim tarball	1m 27s
x86_64-linux: with shared libraries	26m 26s
x86_64-darwin: with shared libraries	40m 33s
aarch64-darwin: with shared libraries	31m 52s
aarch64-linux: with shared libraries	22m 21s
	2h 2m 39s

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