docs: document status-dependent action attempt values

[razor-x]

Follow-up to #632: documents the (action_type, status) action attempt classes in the README's Action Attempts section — how error and result are None outside their status, with a narrow-on-status example:

if action_attempt.status == "success":
    print(action_attempt.result)  # The result is not None here.

if action_attempt.status == "error":
    print(action_attempt.error.message)  # The error is not None here.

Also notes that waiting returns a SuccessActionAttempt. README-only change; rstcheck passes.

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