Fix doc build on unsupported oses

Author: devnexen

library/std/src/os/unix/net/datagram.rs

@@ -840,7 +840,14 @@ impl UnixDatagram {
 
     /// Set the id of the socket for network filtering purpose
     ///
-    /// ```no_run
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
     /// #![feature(unix_set_mark)]
     /// use std::os::unix::net::UnixDatagram;
     ///

library/std/src/os/unix/net/stream.rs

@@ -426,7 +426,14 @@ impl UnixStream {
 
     /// Set the id of the socket for network filtering purpose
     ///
-    /// ```no_run
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
     /// #![feature(unix_set_mark)]
     /// use std::os::unix::net::UnixStream;
     ///