Missing documentation for errno

[Patrick Rapin <toupie300@...>]

I just wanted to test the behavior of os.remove on Windows compared to Linux.
And I discovered something.
In case of errors, functions os.remove and os.rename return 3 values:
 1. nil
 2. error message (string)
 3. errno (number)

But looking at the documentation, neither the 5.1.4 nor the 5.2.0
version mention the extra errno return value.
Is that a hidden feature or an omission ?