Summary

• add guidance to download_fileobj docstrings (client, bucket, object) about calling flush() and seek(0) before reading from the same file object
• update the S3 downloading guide with a concrete download_fileobj example showing flush() + seek(0) in a wb+ context

Why

Issue #1304 highlights that managed multipart downloads can leave the writable file object in a state where immediate reads may appear incomplete unless callers flush and reset the cursor. This PR documents that usage pattern where users are most likely to look for it.

Validation

• python -m pytest tests/unit/s3/test_inject.py -q
• python -m ruff check boto3/s3/inject.py

(Attempted sphinx -b dummy docs build locally, but full dummy build stalled in this environment; changes are limited to docstrings and one guide page.)

Fixes #1304