Mention relation between DOMAIN and mail settings in .env template

.env.template

@@ -69,7 +69,8 @@
 
 ## Domain settings
 ## The domain must match the address from where you access the server
-## Unless you are using U2F, or having problems with attachments not downloading, there is no need to change this
+## It's recommended to configure this value, otherwise certain functionality might not work,
+## like attachment downloads, email links and U2F.
 ## For U2F to work, the server must use HTTPS, you can use Let's Encrypt for free certs
 # DOMAIN=https://bw.domain.tld:8443
 
@@ -88,6 +89,7 @@
 # ROCKET_TLS={certs="/path/to/certs.pem",key="/path/to/key.pem"}
 
 ## Mail specific settings, set SMTP_HOST and SMTP_FROM to enable the mail service.
+## To make sure the email links are pointing to the correct host, set the DOMAIN variable.
 ## Note: if SMTP_USERNAME is specified, SMTP_PASSWORD is mandatory
 # SMTP_HOST=smtp.domain.tld
 # SMTP_FROM=bitwarden-rs@domain.tld