list: Fix docs for APPEND to show that elements are optional

list(APPEND) has been able to append nothing since commit a06dcdba
(Allow LIST(APPEND) command to append nothing, 2008-01-16) but the
documentation still used to imply that at least one argument is
required.
Merge topic 'doc-list-append-nothing' into next

eec7834 list: Fix docs for APPEND to show that elements are optional