Improve documentation of error.* fields

[ehmicky]

This PR documents the error-related fields as error.* instead of result.*.

Technically, on success, result.timedOut, result.isCanceled, result.isMaxBuffer and result.isTerminated are false, and result.exitCode is 0. However, those are defined on success mostly for orthogonality reasons.

Users should instead check whether the subprocess failed first. If not, they do not need to check those properties. They can check whether the subprocess failed by using reject: true (the default value), error.failed, or error instanceof ExecaError.

Documenting those fields as error.* makes it clearer that those are intended for subprocess failures.