Clarify S3FileSystem Credentials chain for EC2

The C implementation of `DefaultAWSCredentialsProviderChain` not only reads the environment variable when trying to resolve AWS credentials, but also looks at profile config and the EC2 Instance Metadata Service.

I want to document this behavior in the documentation.
The `s3fs` documentation mentions the specific case for EC2

docs/source/python/filesystems.rst

@@ -153,8 +153,9 @@ PyArrow implements natively a S3 filesystem for S3 compatible storage.
 The :class:`S3FileSystem` constructor has several options to configure the S3
 connection (e.g. credentials, the region, an endpoint override, etc). In
 addition, the constructor will also inspect configured S3 credentials as
-supported by AWS (for example the ``AWS_ACCESS_KEY_ID`` and
-``AWS_SECRET_ACCESS_KEY`` environment variables).
+supported by AWS (such as the ``AWS_ACCESS_KEY_ID`` and
+``AWS_SECRET_ACCESS_KEY`` environment variables, AWS configuration files,
+and EC2 Instance Metadata Service for EC2 nodes).
 
 
 Example how you can read contents from a S3 bucket::

python/pyarrow/_s3fs.pyx

@@ -140,14 +140,20 @@ cdef class S3FileSystem(FileSystem):
  """
  S3-backed FileSystem implementation
 
- If neither access_key nor secret_key are provided, and role_arn is also not
- provided, then attempts to initialize from AWS environment variables,
- otherwise both access_key and secret_key must be provided.
+ AWS access_key and secret_key can be provided explicitly.
 
  If role_arn is provided instead of access_key and secret_key, temporary
  credentials will be fetched by issuing a request to STS to assume the
  specified role.
 
+ If neither access_key nor secret_key are provided, and role_arn is also not
+ provided, then attempts establish the credentials automatically.
+ S3FileSystem will try the following methods, in order:
+
+ * `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY`, and `AWS_SESSION_TOKEN` environment variables
+ * configuration files such as `~/.aws/credentials` and `~/.aws/config`
+ * for nodes on Amazon EC2, the EC2 Instance Metadata Service
+
  Note: S3 buckets are special and the operations available on them may be
  limited or more expensive than desired.