Add warning to set SENDMAIL_ARGS to -- (#19102)

Add warning to set SENDMAIL_ARGS to --  (#19102)

Even with #17688 email addresses that contain an initial `-` may still be present in the db and it may in future still be possible to imagine a situation whereby initial `-` are repermitted.

This PR simply updates the documentation to warn users to set their SENDMAIL_ARGS with a terminal `--` to prevent this possibility email addresses being interpreted as options.

Signed-off-by: Andrew Thornton <art27@cantab.net>