rustdoc: description of submodules are displayed twice

[omasanori]

In the document of std::io, description of submodules are repeated twice. Also, in the submodules' doc (e.g. std::io::fs' one), the description written in libstd/io/mod.rs is repeated twice, and then the description written in the submodule follows them.

I think the repetition is obviously a bug. However, I'm unsure whether it is also a bug to insert parent's description into children even though children has its own description or not.