Python: Clarify docs on environment variables (#165)

mkdocs/docs/configuration.md

@@ -32,13 +32,19 @@ There are three ways to pass in configuration:
 - Through environment variables
 - By passing in credentials through the CLI or the Python API
 
-The configuration file is recommended since that's the most transparent way. If you prefer environment configuration:
+The configuration file is recommended since that's the easiest way to manage the credentials.
+
+Another option is through environment variables:
 
 ```sh
 export PYICEBERG_CATALOG__DEFAULT__URI=thrift://localhost:9083
+export PYICEBERG_CATALOG__DEFAULT__S3__ACCESS_KEY_ID=username
+export PYICEBERG_CATALOG__DEFAULT__S3__SECRET_ACCESS_KEY=password
 ```
 
-The environment variable picked up by Iceberg starts with `PYICEBERG_` and then follows the yaml structure below, where a double underscore `__` represents a nested field.
+The environment variable picked up by Iceberg starts with `PYICEBERG_` and then follows the yaml structure below, where a double underscore `__` represents a nested field, and the underscore `_` is converted into a dash `-`.
+
+For example, `PYICEBERG_CATALOG__DEFAULT__S3__ACCESS_KEY_ID`, sets `s3.access-key-id` on the `default` catalog.
 
 ## FileIO
 
@@ -80,8 +86,6 @@ For the FileIO there are several configuration options available:
 
 ### Azure Data lake
 
-### Azure Data lake
-
 | Key                     | Example                                                                                   | Description                                                                                                                                                                                                                                                                            |
 | ----------------------- | ----------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | adlfs.connection-string | AccountName=devstoreaccount1;AccountKey=Eby8vdM02xNOcqF...;BlobEndpoint=http://localhost/ | A [connection string](https://learn.microsoft.com/en-us/azure/storage/common/storage-configure-connection-string). This could be used to use FileIO with any adlfs-compatible object storage service that has a different endpoint (like [azurite](https://github.com/azure/azurite)). |