Rollup merge of #28754 - luser:fix-rustdoc-module-links, r=Manishearth

The links in the rustdoc for several places in fmt were trying to link to
the std::fmt module but actually linking to std, which was confusing.
While trying to figure out why I noticed that the documentation chapter of
the Rust book has examples that show this same bug (although it doesn't seem
widespread in practice).

r? @steveklabnik