Clarify that CString::from_vec_unchecked appends 0 byte.

library/std/src/ffi/c_str.rs

@@ -409,6 +409,8 @@ impl CString {
     /// Creates a C-compatible string by consuming a byte vector,
     /// without checking for interior 0 bytes.
     ///
+    /// Trailing 0 byte will be appended by this function.
+    ///
     /// This method is equivalent to [`CString::new`] except that no runtime
     /// assertion is made that `v` contains no 0 bytes, and it requires an
     /// actual byte vector, not anything that can be converted to one with Into.