B2 SDK docs: document the new SSE-B2 bucket and file-write defaults

[ffumero2003]

Description

The encryption-defaults branch changes omitted bucket encryption to SSE-B2 and rejects explicit plaintext settings for bucket defaults and file writes. The current encryption guide still describes no encryption as a valid bucket-default outcome. The behavioral change needs user-facing documentation and migration guidance before release.

Steps to reproduce

bucket = b2_api.create_bucket('example-bucket', 'allPrivate')
file_version = bucket.upload_bytes(b'data', 'example.txt')

# New behavior: file_version.server_side_encryption is SSE_B2_AES.
# Explicit encryption=SSE_NONE is rejected for a file write.

Expected behavior

The encryption guide should distinguish omitted encryption, SSE-B2, and SSE-C; explain that omission delegates to the bucket default; and state where SSE_NONE is no longer accepted.

Actual behavior

The current guide says the bucket default may result in no encryption and provides no migration guidance for the new behavior.

Environment

• Python version: Any supported version
• b2sdk version: origin/encryption-defaults at 63727e89
• OS: Any

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