doc: align Buffer.concat documentation with behavior #2948

Job	Run time
Build slim tarball	1m 45s
aarch64-darwin: with shared libraries	2h 20m 40s
aarch64-linux: with shared libraries	1h 25m 59s
x86_64-darwin: with shared libraries	1h 18m 53s
x86_64-linux: with shared libraries	2h 4m 52s
	7h 12m 9s

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