doc: Makecache with timer tries only one mirror

Related: https://bugzilla.redhat.com/show_bug.cgi?id=922667
Related: https://issues.redhat.com/browse/RHEL-1342

doc/command_ref.rst

@@ -972,10 +972,11 @@ Makecache Command
 
 ``dnf [options] makecache --timer``
  Like plain ``makecache``, but instructs DNF to be more resource-aware,
- meaning it will not do anything if running on battery power and will terminate
+ meaning it will not do anything if running on battery power, it will terminate
  immediately if it's too soon after the last successful ``makecache`` run
  (see :manpage:`dnf.conf(5)`, :ref:`metadata_timer_sync
- <metadata_timer_sync-label>`).
+ <metadata_timer_sync-label>`), and if the first mirror in a repository mirrorlist fails,
+ it will not try to synchronize the metadata from more mirrors for that repository.
 
 .. _mark_command-label: