Add safety disclaimer for ::from_name() also

Author: Árni Dagur

src/shadow.rs

@@ -101,6 +101,13 @@ impl From<&libc::spwd> for Shadow {
 
 impl Shadow {
     /// Gets a [`Shadow`] entry for the given username, or returns [`None`].
+    ///
+    /// # Safety
+    /// Care should be taken when this function is used in different threads
+    /// without synchronization. This is because the underlying function used
+    /// ([`getspnam()`][1]) is not thread safe.
+    ///
+    /// [1]: http://man7.org/linux/man-pages/man3/shadow.3.html
     pub fn from_name(user: &str) -> Option<Shadow> {
         let c_user = CString::new(user).unwrap();