docs: Remove "experimental" notices for now-stable features

Alias `*` destinations, "via" aliases, and the max_queue_items and
give_up_send_after options have been out for more than a year, and a few
releases, with no significant concerns.

This patch removes the "experimental" notices from the documentation
accordingly.

etc/chasquid/chasquid.conf

@@ -102,14 +102,12 @@ submission_over_tls_address: ":465"
 #haproxy_incoming: false
 
 # Maximum number of items in the queue.
-# EXPERIMENTAL - Might change in backwards-incompatible ways.
 # If we have this many items in the queue, we reject new incoming email. Be
 # careful when increasing this, as we keep all items in memory.
 # Default: 200 (but may change in the future).
 #max_queue_items: 200
 
 # How long do we keep retrying sending an email before we give up.
-# EXPERIMENTAL - Might change in backwards-incompatible ways.
 # Once we give up, a DSN will be sent back to the sender.
 # The format is a Go duration string (e.g. "48h" or "360m"; note days are not
 # a supported unit).