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>

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