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GH-35409: [Python][Docs] Clarify S3FileSystem Credentials chain for EC2 (#35312)



### Rationale for this change

When resolving AWS credentials on EC2 hosts, the underlying AWS SDK also looks at the EC2 Instance Metadata Service. 

I want to document this behavior for `pyarrow`.  The [`s3fs` documentation](https://s3fs.readthedocs.io/en/latest/#credentials) mention this specific case for EC2.

### What changes are included in this PR?

Documentation for the behavior described above. 

#### Technical Details
`S3FileSystem` uses the [`CS3Options.Defaults()`](https://github.com/apache/arrow/blob/5de56928e0fe43f02005552eee058de57ffb2682/python/pyarrow/_s3fs.pyx#L317) option when no credentials are passed into the constructor.  It utilizes the [`Aws::Auth::DefaultAWSCredentialsProviderChain`](https://github.com/apache/arrow/blob/1de159d0f6763766c19b183dd309b8757723b43a/cpp/src/arrow/filesystem/s3fs.cc#L213)

The C++ implementation of [`DefaultAWSCredentialsProviderChain`](https://sdk.amazonaws.com/cpp/api/0.14.3/class_aws_1_1_auth_1_1_default_a_w_s_credentials_provider_chain.html) not only [reads the environment variable](https://sdk.amazonaws.com/cpp/api/0.14.3/class_aws_1_1_auth_1_1_environment_a_w_s_credentials_provider.html) when trying to resolve AWS credentials, but also [looks at profile config](https://sdk.amazonaws.com/cpp/api/0.14.3/class_aws_1_1_auth_1_1_profile_config_file_a_w_s_credentials_provider.html) and the [EC2 Instance Metadata Service](https://sdk.amazonaws.com/cpp/api/0.14.3/class_aws_1_1_auth_1_1_instance_profile_credentials_provider.html). 

### Are these changes tested?

No, just documentation changes

### Are there any user-facing changes?

Yes, changing public documentation

* Closes: #35409

### Render Changes
Render the changes locally via [Building the doc](https://arrow.apache.org/docs/developers/documentation.html#building-docs): 
`docs/source/python/filesystems.rst`:
![Screenshot 2023-07-30 at 6 22 02 PM](https://github.com/apache/arrow/assets/9057843/6af053a3-e7a7-4a68-a5b5-02c50e9290c6)

`python/pyarrow/_s3fs.pyx`:
![Screenshot 2023-07-31 at 3 31 30 PM](https://github.com/apache/arrow/assets/9057843/d79768be-67ce-46c0-88ed-a833e540f77d)

Lead-authored-by: Kevin Liu <kevinjqliu@users.noreply.github.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>