rust: error: improve `Error::from_errno` documentation

rust: error: improve `Error::from_errno` documentation

This constructor is public since commit 5ed147473458 ("rust: error:
make conversion functions public"), and we will refer to it from the
documentation of `to_result` in a later commit.

Thus improve its documentation, including adding examples.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>