Document that BodyState.STOP may carry the final bytes

[hyperxpro]

Motivation:

PR #2232 changed MultipartBody.transferTo() to return STOP on the same call that writes the body's final bytes, meaning STOP may still be accompanied by unread data in the target buffer. However, BodyState.STOP is currently documented as meaning "nothing to read," and AuthenticatorUtils's auth-int loop assumes this behavior, which would truncate such a body (currently unreachable because multipart bodies do not use this path).

Modification:

Update the Body.transferTo() and BodyState.STOP documentation to clarify that STOP may accompany the final bytes and that consumers must drain the target buffer before stopping. Add a comment in the auth-int loop documenting its assumption.

Result:

The STOP-with-data contract is explicitly documented, and the existing assumption in AuthenticatorUtils is clearly identified. Documentation only; no behavioral changes.