[mail] Clarify recipient parameter in rule action (#6211)

Signed-off-by: Jan N. Klug <jan.n.klug@rub.de>

bundles/org.openhab.binding.mail/README.md

@@ -98,6 +98,8 @@ Six different actions available:
 The `sendMail(...)` send a plain text mail (with attachments if supplied).
 The `sendHtmlMail(...)` send a HTML mail (with attachments if supplied).
 
+`recipient` can be a single address (`mail@example.com`) or a list of addresses, concatenated by a comma (`mail@example.com, mail2@example.com`).
+
 Since there is a separate rule action instance for each `smtp` thing, this needs to be retrieved through `getActions(scope, thingUID)`.
 The first parameter always has to be `mail` and the second is the full Thing UID of the SMTP server that should be used.
 Once this action instance is retrieved, you can invoke the action method on it.