Add a link to Erlang application specification, closes #13042

Author: josevalim

lib/elixir/lib/application.ex

@@ -200,7 +200,7 @@ defmodule Application do
   In the sections above, we have configured an application in the
   `application/0` section of the `mix.exs` file. Ultimately, Mix will use
   this configuration to create an [*application resource
-  file*](https://www.erlang.org/doc/man/application.html), which is a file called
+  file*](https://www.erlang.org/doc/man/app), which is a file called
   `APP_NAME.app`. For example, the application resource file of the OTP
   application `ex_unit` is called `ex_unit.app`.
 
@@ -467,6 +467,9 @@ defmodule Application do
 
     * #{Enum.map_join(@application_keys, "\n  * ", &"`#{inspect(&1)}`")}
 
+  For a description of all fields, see [Erlang's application
+  specification](https://www.erlang.org/doc/man/app).
+
   Note the environment is not returned as it can be accessed via
   `fetch_env/2`. Returns `nil` if the application is not loaded.
   """

lib/mix/lib/mix/tasks/compile.app.ex

@@ -78,8 +78,10 @@ defmodule Mix.Tasks.Compile.App do
       technically valid in any resource file, but it is only effective for
       applications with a callback module. Defaults to `:infinity`.
 
-  Besides the options above, `.app` files also expect other options like
-  `:modules` and `:vsn`, but these are automatically added by Mix.
+  Besides the options above, `.app` files also expect other options
+  like `:modules` and `:vsn`, but these are automatically added by Mix.
+  The complete list can be found on [Erlang's application
+  specification](https://www.erlang.org/doc/man/app).
 
   ## Command line options