FreeBSD: hide ucontext from docs to fix xdoc build

Why does it try to document the cfg'd out struct?

+ cargo xdoc --target i686-unknown-freebsd --no-default-features --features extra_traits
error[E0412]: cannot find type `ucontext_t` in this scope
   --> src/unix/bsd/freebsdlike/freebsd/mod.rs:166:16
    |
166 |     pub struct ucontext_t {
    |                ^^^^^^^^^^ not found in this scope

Author: valpackett

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -163,6 +163,7 @@ s_no_extra_traits! {
 
     #[cfg_attr(feature = "extra_traits", derive(Debug))]
     #[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
+    #[doc(hidden)] // rustdoc tries to generate for i686, can't find it
     pub struct ucontext_t {
         pub uc_sigmask: ::sigset_t,
         pub uc_mcontext: ::mcontext_t,