docs(env): update OVERRIDE_ADMIN_EMAIL documentation

- Clarify the behavior of the OVERRIDE_ADMIN_EMAIL environment variable
- Emphasize the security benefits of this feature
- Provide more detailed explanations for different scenarios

Author: fulleni

.env.example

@@ -34,9 +34,12 @@
 # Use "https://api.eu.sendgrid.com" for EU-based accounts.
 # SENDGRID_API_URL="https://api.sendgrid.com"
 
-# REQUIRED FOR FIRST RUN: The email address for the initial administrator account.
-# This value is used ONLY on the first startup to create the first admin user.
-# If a user with this email already exists, nothing will happen.
-# Changing this value after the first run WILL NOT change the existing admin's email.
-# To create a new admin, you must do so from the dashboard after logging in.
-# INITIAL_ADMIN_EMAIL="[email protected]"
+# ADMIN OVERRIDE: Sets the single administrator account for the application.
+# On server startup, the system ensures that the user with this email is the
+# one and only administrator.
+# - If no admin exists, one will be created with this email.
+# - If an admin with a DIFFERENT email exists, they will be REMOVED and
+#   replaced by a new admin with this email.
+# - If an admin with this email already exists, nothing changes.
+# This provides a secure way to set or recover the admin account.
+# OVERRIDE_ADMIN_EMAIL="[email protected]"