minor #9824 Add URL-encoding notice (nebkam, javiereguiluz)

This PR was merged into the 4.0 branch.

Discussion
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Add URL-encoding notice

I've had trouble connecting to Amazon SES via recommended method.
I kept getting:
> Exception occurred while flushing email queue: Failed to authenticate on SMTP server with username "***"
I've traced it to "+" sign in the password that Amazon SES automatically assigned to my SMTP credentials.
I had a hunch that special characters might be a problem and was thinking along the lines of wrapping the whole MAILER_URL value in single quotes.
Then I ran into https://stackoverflow.com/a/50498999/1419874 that solved the issue for me too.
Since the OP lost 3hrs and myself almost an hour, I think URL encoding might not be so obvious.
Hence I propose this change. I've intentionally put it at the top, since I think it applies to any method that has special characters in username/password, not just Amazon SES.

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Commits
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7f72a6f Reword
61a69ac Add URL-encoding notice

Author: javiereguiluz

email.rst

@@ -37,7 +37,9 @@ environment variable in the ``.env`` file:
     # use this to disable email delivery
     MAILER_URL=null://localhost
 
-    # use this to configure a traditional SMTP server
+    # use this to configure a traditional SMTP server (make sure to URL-encode the
+    # values of the username and password if they contain non-alphanumeric characters
+    # such as '+', '@', ':' and '*', which are reserved in URLs)
     MAILER_URL=smtp://localhost:25?encryption=ssl&auth_mode=login&username=&password=
 
 Refer to the :doc:`SwiftMailer configuration reference </reference/configuration/swiftmailer>`