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ARROW-11168: [Rust] [Doc] Fix cargo doc warnings

The PR fixed cargo doc warnings (including the generated flatbuffer files), and corrected several typos.
To catch all possible cargo doc warnings, it looks like we have to run `cargo doc` for both nightly and stable.

I had filed an issue https://issues.apache.org/jira/browse/ARROW-11179 to address the cargo doc problem about generated flatbuffer files, hope this can be resolved soon. Currently, the patch is created as `format-0ed34c83.patch` for regen.sh.

Closes #9133 from mqy/ARROW-11168_fix_cargo_warnings

Authored-by: mqy <meng.qingyou@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>