root-5.30 was released, so documentation should be updated as well. It is sufficient to rename root-docs-5.28-r1.ebuild.
I added this package into science overlay. According to the discussion about large tarball at bug 348968: I see two solutions: 1) restrict mirror; 2) generate html docs in the root ebuild. The latter has one severe issue: an internet access is required during documentation generation (at least the access for svn logs), this is fixable, but non-trivial...
Fixed now.