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 rust-lang#3538`.

Since rust-lang#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`.

Author: lkuper

src/libextra/std.rc

@@ -123,7 +123,7 @@ pub mod serialize;
 // A curious inner-module that's not exported that contains the binding
 // 'extra' so that macro-expanded references to std::serialize and such
 // can be resolved within libextra.
-#[doc(hidden)] // FIXME #3538
+#[doc(hidden)]
 pub mod std {
     pub use serialize;
     pub use test;
@@ -134,7 +134,7 @@ pub mod std {
     pub use core::cmp;
     pub use core::sys;
 }
-#[doc(hidden)] // FIXME #3538
+#[doc(hidden)]
 pub mod extra {
     pub use serialize;
     pub use test;

src/libstd/num/cmath.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[doc(hidden)]; // FIXME #3538
-
 // function names are almost identical to C's libmath, a few have been
 // renamed, grep for "rename:"
 

src/libstd/stackwalk.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[doc(hidden)]; // FIXME #3538
-
 use cast::transmute;
 use unstable::intrinsics;
 

src/libstd/task/local_data_priv.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[doc(hidden)]; // FIXME #3538
-
 use cast;
 use cmp::Eq;
 use libc;

src/libstd/task/mod.rs

@@ -195,7 +195,6 @@ pub fn task() -> TaskBuilder {
     }
 }
 
-#[doc(hidden)] // FIXME #3538
 priv impl TaskBuilder {
     fn consume(&mut self) -> TaskBuilder {
         if self.consumed {

src/libstd/task/rt.rs

@@ -14,7 +14,7 @@ The task interface to the runtime
 
 */
 
-#[doc(hidden)]; // FIXME #3538
+#[doc(hidden)];
 
 use libc;
 

src/libstd/task/spawn.rs

@@ -70,7 +70,7 @@
  *
  ****************************************************************************/
 
-#[doc(hidden)]; // FIXME #3538
+#[doc(hidden)];
 
 use cast::transmute;
 use cast;

src/libstd/unicode.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[doc(hidden)]; // FIXME #3538
-
 // The following code was generated by "src/etc/unicode.py"
 
 pub mod general_category {