when not defined, not allowed

lib/basic_signup_whitelist.ex

@@ -1,10 +1,18 @@
 defmodule BasicSignupWhitelist do
   @moduledoc """
-  Documentation for `BasicSignupWhitelist`.
+  Checks if an email address is allowed based on the SIGNUP_ALLOWED_EMAILS env variable.
   """
 
   @doc """
-  false
+  Checks if an email address is allowed based on the SIGNUP_ALLOWED_EMAILS env variable.
+
+  Rules:
+    - Returns true if ALLOWED_EMAILS == "*" (all emails allowed)
+    - Returns true if email matches any entry in the comma-separated list
+    - Returns false if:
+        * Environment variable doesn't exist
+        * Environment variable is empty string
+        * Email not found in whitelist
 
   ## Examples