doc: give attr.tree a bit more visibility

In "git help config" output, attr.tree mentions both --attr-source
and GIT_ATTR_SOURCE, but the description of --attr-source and
GIT_ATTR_SOURCE that appear in "git help git", attr.tree is missing.

Add it so that these three are described together in both places.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/git.txt

@@ -224,7 +224,10 @@ If you just want to run git as if it was started in `<path>` then use
 --attr-source=<tree-ish>::
 	Read gitattributes from <tree-ish> instead of the worktree. See
 	linkgit:gitattributes[5]. This is equivalent to setting the
-	`GIT_ATTR_SOURCE` environment variable.
+	`GIT_ATTR_SOURCE` environment variable.  The `attr.tree`
+	configuration variable is used as a fallback when this option
+	or the environment variable are not in use.
+
 
 GIT COMMANDS
 ------------