make env validation errors clear

* modify mail smtp variable names

.env.example

@@ -1,16 +1,19 @@
+# your domain, e.g https://example.com
 APP_URL=http://localhost:3000
-APP_SECRET=REPLACE_WITH_LONG_SECRET
 PORT=3000
 
+# make sure to replace this.
+APP_SECRET=REPLACE_WITH_LONG_SECRET
+
 JWT_TOKEN_EXPIRES_IN=30d
 
 DATABASE_URL="postgresql://postgres:password@localhost:5432/docmost?schema=public"
 REDIS_URL=redis://127.0.0.1:6379
 
-# local | s3
+# options: local | s3
 STORAGE_DRIVER=local
 
-# S3 Config
+# S3 driver config
 AWS_S3_ACCESS_KEY_ID=
 AWS_S3_SECRET_ACCESS_KEY=
 AWS_S3_REGION=
@@ -18,15 +21,17 @@ AWS_S3_BUCKET=
 AWS_S3_ENDPOINT=
 AWS_S3_URL=
 
-# EMAIL drivers: smtp / postmark / log
+# options: smtp | postmark
 MAIL_DRIVER=smtp
-MAIL_HOST=127.0.0.1
-MAIL_PORT=2525
-MAIL_USERNAME=
-MAIL_PASSWORD=
 MAIL_FROM_ADDRESS=hello@example.com
 MAIL_FROM_NAME=Docmost
 
-# for postmark driver
+# SMTP driver config
+SMTP_HOST=127.0.0.1
+SMTP_PORT=587
+SMTP_USERNAME=
+SMTP_PASSWORD=
+
+# Postmark driver config
 POSTMARK_TOKEN=