Avoid making the start of a doc code block conditional

Placing the opening triple-backquote inside a `cfg_attr` makes many
tools confused, including syntax highlighters (e.g. vim's) and rustfmt.

Instead, use a `cfg` inside the doc code block.

Author: joshtriplett

library/std/src/path.rs

@@ -3259,15 +3259,16 @@ impl Path {
     ///
     /// # Examples
     ///
-    #[cfg_attr(unix, doc = "```no_run")]
-    #[cfg_attr(not(unix), doc = "```ignore")]
+    /// ```rust,no_run
+    /// # #[cfg(unix)] {
     /// use std::path::Path;
     /// use std::os::unix::fs::symlink;
     ///
     /// let link_path = Path::new("link");
     /// symlink("/origin_does_not_exist/", link_path).unwrap();
     /// assert_eq!(link_path.is_symlink(), true);
     /// assert_eq!(link_path.exists(), false);
+    /// # }
     /// ```
     ///
     /// # See Also