Simplify instructions for exporting with docker

The export workflow reusing the `/consume` volume is complex and error-
prone, and not at all necessary if the `docker-compose.yml` file has a
volume for `/export` from the beginning.

docker-compose.yml.example

@@ -23,6 +23,12 @@ services:
             # You have to adapt the local path you want the consumption
             # directory to mount to by modifying the part before the ':'.
             - /path/to/arbitrary/place:/consume
+            # Likewise, you can add a local path to mount a directory for
+            # exporting. This is not strictly needed for paperless to
+            # function, only if you're exporting your files: uncomment
+            # it and fill in a local path if you know you're going to 
+            # want to export your documents.
+            # - /path/to/another/arbitrary/place:/export
         env_file: docker-compose.env
         command: ["document_consumer"]