Final answer:
The Credit Agreement is the document that discloses the interest rate and terms for computation in a margin account.
Step-by-step explanation:
The document that discloses the interest rate charged by the broker-dealer, including the method of interest computation and the situations under which interest rates may change when opening a margin account is the Credit Agreement. Further this agreement outlines the terms and details of the loan provided by the broker to the investor when purchasing securities on margin. It is crucial to understand the specifics in this document, as it governs how and when the borrowed funds need to be repaid and under what conditions the interest rates could be adjusted.