doc: distinguish between `fs.Stats` and `fs.BigIntStats` #1032

Job	Run time
Build slim tarball	1m 39s
aarch64-linux: with shared libraries	24m 37s
x86_64-darwin: with shared libraries	47m 43s
aarch64-darwin: with shared libraries	31m 32s
x86_64-linux: with shared libraries	29m 13s
	2h 14m 44s

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