The current ebuild of llvm always build documentation and so the doc USE flag is not used. This bug is created for https://github.com/gentoo/gentoo/pull/17837
USE=doc is used for HTML documentation.
According to metadata.xml doc is for: <flag name="doc">Build and install the HTML documentation and regenerate the man pages</flag> So please check MR, the part which is commented is "docs-llvm-html" and docs-llvm-*man, so I think it is perfectly valid.
No, it's not.