chore: apply credo suggestions (#3)

Mainly add module docs and fix some aliases.

lib/wabanex/users/create.ex

@@ -1,4 +1,7 @@
 defmodule Wabanex.Users.Create do
+  @moduledoc """
+  Context for user creation.
+  """
   alias Wabanex.{Repo, User}
 
   def call(params) do

lib/wabanex/users/get.ex

@@ -1,4 +1,7 @@
 defmodule Wabanex.Users.Get do
+  @moduledoc """
+  Context to fetch a given user and its current training.
+  """
   import Ecto.Query
 
   alias Wabanex.{Repo, Training, User}