Document that username/password in SMTP and database config must be urlencoded

Some characters are unsafe to use in configuration variables that are URLs.

For instance, EMAIL_CONFIG=smtp+ssl://noreply@domain.net:password@smtp.domain.net:465 will not work as expected because the username is an email, and @ isn't a safe char for use in URLs.

Such usernames and passwords must be urlencoded, for instance using the snippet below:

python3 -c 'import urllib.parse; print(urllib.parse.quote_plus("noreply@domain.net"))'
noreply%40domain.net

cf https://github.com/joke2k/django-environ#using-unsafe-characters-in-urls