add stronger warning to CString::from_raw

Author: durka

src/libstd/ffi/c_str.rs

@@ -225,8 +225,11 @@ impl CString {
     /// Retakes ownership of a `CString` that was transferred to C.
     ///
     /// This should only ever be called with a pointer that was earlier
-    /// obtained by calling `into_raw` on a `CString`. Additionally, the length
-    /// of the string will be recalculated from the pointer.
+    /// obtained by calling `into_raw` on a `CString`. In particular, using this method
+    /// to create a `CString` pointing at memory that will be freed by other code
+    /// (such as a C library) will lead to undefined behavior!
+    /// 
+    /// Additionally, the length of the string will be recalculated from the pointer.
     #[stable(feature = "cstr_memory", since = "1.4.0")]
     pub unsafe fn from_raw(ptr: *mut c_char) -> CString {
         let len = libc::strlen(ptr) + 1; // Including the NUL byte