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>

custom/conf/app.example.ini

@@ -1533,6 +1533,7 @@ PATH =
 ;SENDMAIL_PATH = sendmail
 ;;
 ;; Specify any extra sendmail arguments
+;; WARNING: if your sendmail program interprets options you should set this to "--" or terminate these args with "--"
 ;SENDMAIL_ARGS =
 ;;
 ;; Timeout for Sendmail

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -666,7 +666,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
    - Enabling dummy will ignore all settings except `ENABLED`, `SUBJECT_PREFIX` and `FROM`.
 - `SENDMAIL_PATH`: **sendmail**: The location of sendmail on the operating system (can be
    command or full path).
-- `SENDMAIL_ARGS`: **_empty_**: Specify any extra sendmail arguments.
+- `SENDMAIL_ARGS`: **_empty_**: Specify any extra sendmail arguments. (NOTE: you should be aware that email addresses can look like options - if your `sendmail` command takes options you must set the option terminator `--`)
 - `SENDMAIL_TIMEOUT`: **5m**: default timeout for sending email through sendmail
 - `SENDMAIL_CONVERT_CRLF`: **true**: Most versions of sendmail prefer LF line endings rather than CRLF line endings. Set this to false if your version of sendmail requires CRLF line endings.
 - `SEND_BUFFER_LEN`: **100**: Buffer length of mailing queue. **DEPRECATED** use `LENGTH` in `[queue.mailer]`

docs/content/doc/usage/email-setup.en-us.md

@@ -35,6 +35,7 @@ ENABLED       = true
 FROM          = gitea@mydomain.com
 MAILER_TYPE   = sendmail
 SENDMAIL_PATH = /usr/sbin/sendmail
+SENDMAIL_ARGS = "--" ; most "sendmail" programs take options, "--" will prevent an email address being interpreted as an option.
 ```
 
 ## Using SMTP