Updating the download config docs to indicate how to point to the download file (#1371)

docs/content/configuration/download.md

@@ -4,9 +4,14 @@ description: What to do when a module is not in storage
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-Athens accepts an HCL formatted Download File that serves as the source of truth for answering the following question: 
+Athens accepts an HCL formatted Download File that has instructions for how it should behave when a module@version isn't found in its storage.
 
-### What should Athens do when a module@version is not found in storage? 
+You configure this download file by setting the `ATHENS_DOWNLOAD_MODE` environment variable in one of two ways:
+
+1. Set its value to `file:$FILE_PATH`, where `FILE_PATH` is the path to the HCL file
+1. Set its value to `custom$BASE_64` where `BASE_64` is the base64 encoded HCL file
+
+## What should Athens do when a module@version is not found in storage?
 
 Say a client sends an HTTP request with the path `/github.com/pkg/errors/@v/v0.8.1` and Athens
 does not have this module in storage. Athens will look at the Download File for one of the following Modes:
@@ -59,4 +64,4 @@ If you have limited storage, then it might be a good idea to only persist some m
 
 **Limited resources:**
 
-If you are running Athens with low memory/cpu, then you can redirect all public modules to proxy.golang.org but asynchronously fetch them so that the client does not timeout. At the same time, you can return a 404 for private modules through the `none` mode and let the client (the Go command) fetch private modules directly through `GOPROXY=<athens-url>,direct` 
+If you are running Athens with low memory/cpu, then you can redirect all public modules to proxy.golang.org but asynchronously fetch them so that the client does not timeout. At the same time, you can return a 404 for private modules through the `none` mode and let the client (the Go command) fetch private modules directly through `GOPROXY=<athens-url>,direct`