attr.tree: HEAD:.gitattributes is no longer the default in a bare repo
51441e64 (stop using HEAD for attributes in bare repository by
default, 2024-05-03) has addressed a recent performance regression
by partially reverting a topic that was merged at
26dd307c (Merge
branch 'jc/attr-tree-config', 2023-10-30). But it forgot to update
the documentation to remove the mention of a special case in bare
repositories.
Let's update the document before the update hits the next release.
Signed-off-by: Junio C Hamano <gitster@pobox.com>