Improve Config.Reader docs

lib/elixir/lib/config/reader.ex

@@ -4,11 +4,24 @@ defmodule Config.Reader do
 
   ## As a provider
 
-  `Config.Reader` can also be used as a `Config.Provider`.
-  When used as a provider, it expects a single argument:
-  which the configuration path (as outlined in
-  `t:Config.Provider.config_path/0`) for the configuration
-  to be read and loaded during the system boot.
+  `Config.Reader` can also be used as a `Config.Provider`. When used
+  as a provider, it expects a single argument: the configuration path
+  (as outlined in `t:Config.Provider.config_path/0`) for the file to
+  be read and loaded during the system boot.
+
+  For example, if you expect the target system to have a config file
+  in an absolute path, you can configure your `mix release` as:
+
+      config_providers: [{Config.Reader, "/etc/config.json"}]
+
+  Or if you want to read a custom path inside the release:
+
+      config_provider: [{Config.Reader, {:system, "RELEASE_ROOT", "/config.exs"}}]
+
+  Note by default Mix releases supports runtime configuration via
+  a `config/releases.exs`. If a `config/releases.exs` exists in your
+  application, it is automatically copied inside the release and
+  automatically set as a config provider.
   """
 
   @behaviour Config.Provider

lib/mix/lib/mix/tasks/release.ex

@@ -568,13 +568,14 @@ defmodule Mix.Tasks.Release do
   ### Config providers
 
   Releases also supports custom mechanisms, called config providers, to load
-  any sort of runtime configuration to the system while it boots. For example,
+  any sort of runtime configuration to the system while it boots. For instance,
   if you need to access a vault or load configuration from a JSON file, it
-  can be achieved with config providers. See the `Config.Provider` for more
-  information and a simple example.
+  can be achieved with config providers. The runtime configuration outlined
+  in the previous section, which is handled by the `Config.Reader` provider.
+  See the `Config.Provider` module for more information and more examples.
 
   The following options can be set inside your releases key in your `mix.exs`
-  to control how runtime configuration and config providers work:
+  to control how config providers work:
 
     * `:start_distribution_during_config` - on Erlang/OTP 22+, releases
       only start the Erlang VM distribution features after the config files