std: Hide facade extension traits in docs

These traits are currently all just unstable parts of the facade which are
implementation details for primitives further up the facade. This may make it
more difficult to find what set of methods you get if only linking to libcore,
but for now that's also unstable behavior.

Closes #22025

Author: alexcrichton

src/libcore/char.rs

@@ -139,6 +139,7 @@ pub fn from_digit(num: u32, radix: u32) -> Option<char> {
 // NB: the stabilization and documentation for this trait is in
 // unicode/char.rs, not here
 #[allow(missing_docs)] // docs in libunicode/u_char.rs
+#[doc(hidden)]
 pub trait CharExt {
     fn is_digit(self, radix: u32) -> bool;
     fn to_digit(self, radix: u32) -> Option<u32>;

src/libcore/num/mod.rs

@@ -2476,6 +2476,7 @@ pub enum FpCategory {
 //               `unused_self`. Removing it requires #8888 to be fixed.
 #[unstable(feature = "core",
            reason = "distribution of methods between core/std is unclear")]
+#[doc(hidden)]
 pub trait Float
     : Copy + Clone
     + NumCast

src/libcore/slice.rs

@@ -63,6 +63,7 @@ use raw::Slice as RawSlice;
 
 /// Extension methods for slices.
 #[allow(missing_docs)] // docs in libcollections
+#[doc(hidden)]
 pub trait SliceExt {
     type Item;
 

src/libcore/str/mod.rs

@@ -1496,6 +1496,7 @@ impl<'a, S: ?Sized> Str for &'a S where S: Str {
 
 /// Methods for string slices
 #[allow(missing_docs)]
+#[doc(hidden)]
 pub trait StrExt {
     // NB there are no docs here are they're all located on the StrExt trait in
     // libcollections, not here.