Make sure that the Docs clarify stream length is not always plaintext length

The S3EC attempts to be as "transparent" as possible when applying client-side encryption. For example, PutObject/GetObject "just works". A deeper example is ranged gets - in Java, a ranged get request will apply to the plaintext range, the customer does not need to manually adjust the range to account for encrypted blocks.

However, there is one aspect that is seemingly not possible, which is contentLength of the GetObject stream. It includes the auth tag. Customers need to read the entire thing, which is their original message length + auth tag (or padding in the case of CBC).

This needs to be well-documented.

先从 S3EC GetObject streams 和 contentLength 的文档开始，然后验证 authenticated encryption 和 CBC padding 如何影响客户必须读取的字节数。完成的标准是，文档清楚地区分 stream 长度和原始 plaintext 长度，并说明必须读取完整的 stream。

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