doc: clarify fs.copyFile() symlink behavior #2682

Job	Run time
Build slim tarball	1m 45s
x86_64-linux: with shared libraries	27m 50s
x86_64-darwin: with shared libraries	39m 39s
aarch64-darwin: with shared libraries	30m 19s
aarch64-linux: with shared libraries	22m 44s
	2h 2m 17s

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