Fix the docs on NetBSD.

Author: asomers

src/mount/bsd.rs

@@ -437,13 +437,15 @@ impl<'a> Drop for Nmount<'a> {
 ///
 /// Useful flags include
 /// * `MNT_FORCE` -     Unmount even if still in use.
-/// * `MNT_BYFSID` -    `mountpoint` is not a path, but a file system ID
-///                     encoded as `FSID:val0:val1`, where `val0` and `val1`
-///                     are the contents of the `fsid_t val[]` array in decimal.
-///                     The file system that has the specified file system ID
-///                     will be unmounted.  See
-///                     [`statfs`](crate::sys::statfs::statfs) to determine the
-///                     `fsid`.
+#[cfg_attr(target_os = "freebsd", doc = "
+* `MNT_BYFSID` -    `mountpoint` is not a path, but a file system ID
+                    encoded as `FSID:val0:val1`, where `val0` and `val1`
+                    are the contents of the `fsid_t val[]` array in decimal.
+                    The file system that has the specified file system ID
+                    will be unmounted.  See
+                    [`statfs`](crate::sys::statfs::statfs) to determine the
+                    `fsid`.
+")]
 pub fn unmount<P>(mountpoint: &P, flags: MntFlags) -> Result<()>
     where P: ?Sized + NixPath
 {