[mail] Add recipients list example to documentation (#6210)
[mail] Add recipients list example to documentation (#6210)

Extend sendMail example with recipients list:
mailActions.sendMail("mail1@example.com, mail2@example.com", "Test subject", "This is the mail content.")
Note ',' delimiter. (';' does not work)
Snickermicker/openhab2,Snickermicker/openhab2,Snickermicker/openhab2,Snickermicker/openhab2