Get rid of no-longer-needed #[doc(hidden)] attributes.

There were several old `#[doc(hidden)]` attributes in libstd and
libextra, left over from when rustdoc didn't hide private
definitions, tagged with `FIXME #3538`.

Since #3538 is now closed, I removed the `#[doc(hidden)]` attributes
as well as the FIXMEs, but I left `#[doc(hidden)]` in
libstd/task/spawn.rs and libstd/task/rt.rs since those two are
apparently `pub`, as well as in libextra/std.rc since std/extra is
`pub`.