attr.tree: HEAD:.gitattributes is no longer the default in a bare repo

51441e6 (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 26dd307 (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 <[email protected]>

Author: gitster

Documentation/config/attr.txt

@@ -1,7 +1,6 @@
 attr.tree::
 	A reference to a tree in the repository from which to read attributes,
-	instead of the `.gitattributes` file in the working tree. In a bare
-	repository, this defaults to `HEAD:.gitattributes`. If the value does
-	not resolve to a valid tree object, an empty tree is used instead.
+	instead of the `.gitattributes` file in the working tree. If the value
+	does not resolve to a valid tree object, an empty tree is used instead.
 	When the `GIT_ATTR_SOURCE` environment variable or `--attr-source`
 	command line option are used, this configuration variable has no effect.