rust: error: improve `to_result` documentation

rust: error: improve `to_result` documentation

Core functions like `to_result` should have good documentation.

Thus improve it, including adding an example of how to perform early
returns with it.

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