docs: clarify "branch -l"

This option is mostly useless these days because we turn on
reflogs by default in non-bare repos.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -72,6 +72,8 @@ OPTIONS
 	Create the branch's reflog.  This activates recording of
 	all changes made to the branch ref, enabling use of date
 	based sha1 expressions such as "<branchname>@\{yesterday}".
+	Note that in non-bare repositories, reflogs are usually
+	enabled by default by the `core.logallrefupdates` config option.
 
 -f::
 --force::